Below 30 megahertz, DTMF with FM signal would take a large amount of bandwidth as well as spread spectrum. The only other options left are Pulse modulation, and FSK.

Commonly, Frequency shift keying is used on these bands, which modulates the frequency a bit with data.

Silly memory aid: The question asks about modulation below 30 MHz. FSK is the shortest/smallest answer.

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Forward Error Correction is a means to prevent transmitted data that has been corrupted by noise or signal loss from being misinterpreted at the receiving device. It involves transmitting redundant data bits that the receiving device can use to verify the accuracy of the transmission. A simple form of forward error correction would be transmitting each bit of data three times (a triplet). The receiver uses an algorithm to interpret this data stream averaging the bits in each triplet to correct for noise. It then "decides" what the correct interpretation of the data should be based on that computation. Forward Error Correction does require more bandwidth than Backward Error Correction due to the additional data being sent as well as more processor time. But FEC is preferred for long range, single data transmissions that are subject to noise, as can occur with amateur radio transmissions.

SillyHint: we're all trying to move 'Forward' with our communications :)

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From the FT4 section of the WSJTX user guide:

FT4 is designed for contesting, particularly on the HF bands. Compared with FT8 it is 3.5 dB less sensitive and requires 1.6 times the bandwidth, but it offers the potential for twice the QSO rate. FT4 is not recommended for everyday use.

FT8 has 15 second transmission periods, and half of 15 is 7.5 seconds.

HINT: FT4 requires Fixed Time intervals. Only one answer mentions a fixed time interval.

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Here is an explanation on how the crossed-ellipse display (or station monitor) works with RTTY:

http://www.w7ay.net/site/Applications/cocoaModem/UsersManual/RTTYPage/BasicRTTY/BasicRTTY.html#basictuning

The principle should be the same.

Silly hint: suddenly disappear = selective fading

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Keyboard-to-keyboard operation refers to the situation where you type characters and they're sent as a simple sequence of characters, one after the other. RTTY, PSK31 and MFSK all operate in this manner, which can be thought of almost like alternate forms of machine CW.

PACTOR, on the other hand, is a packet protocol that requires information to be divided up into packets which have header and footer information for features like data compression, error detection and acknowledgement messages for automatic repeat request (ARQ). Therefore it does not support "keyboard-to-keyboard" operation.

Just remember that PACTOR is for packets, not for keyboards typing to each other.

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This data rate allows HF packet transmissions to stay within the bandwidth used by other transmission modes such as SSB speech. This is important on the crowded HF bands.

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If you are transmitting on a frequency your intended recipient is not listening on, naturally they will not receive it. Additionally if you are using a different digital protocol than the intended recipient, they will not understand much the same way speaking in a foreign language would cause confusion. Finally, there may be a station from another direction that they can hear and you can't; this station's signal might be more powerful than yours and would either overpower yours or simply make both of them unintelligible.

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PACTOR is a radio modulation mode used by amateur radio operators, marine radio stations, and radio stations in isolated areas to send and receive digital information via radio. Of the choices presented it is the best mode to transfer binary files. Not to say it cannot be done in other modes.

http://en.wikipedia.org/wiki/PACTOR

Silly Hint: You need to PAC(k) your files before you send them.

Silly Hint 2: PACTOR and binary both have 6 letters.

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Similar to the way Morse Code has different lengths for different characters (E is one dot) PSK31 uses variable length coding where the most used characters are given shorter lengths to save bandwidth.

It was originally called varicode by its inventor, an amateur radio operator.

SILLY HINT: Question has "length" in it. Only two answers have numbers in them that you could use to measure a length. The correct "ONE," has "1" in it - D

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PSK31 operates with a very narrow bandwidth of only 31Hz, with contacts spaced as close as 100Hz apart. As many as 20 conversations can be realistically squeezed into the bandwidth of a single 2.5KHz SSB transmission. [PSK31 Spectrum efficiency on Wikipedia]

MFSK16 uses about 316 Hz bandwidth.

Hint: (PSK) Pretty SKinny

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Amateur transceivers use two different methods to modulate a signal using FSK: direct FSK and audio FSK. The difference between direct FSK and audio FSK is that direct FSK applies the data signal to the transmitter VFO. (E2E11) When using audio FSK, audio, typically from a computer sound card, is used to shift the frequency of the transmitted signal.

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ALE is Automatic Link Establishment, the standard for digitally initiating and sustaining HF radio communications in an automated manner. You can think of it as an automated way of calling for a specific station and answering that call.

When attempting to call another station, an operator enters an ALE address into the radio. The calling radio selects an idle channel (frequency in its list) and transmits call messages. On the radio waiting for a call, ALE constantly scans a list of frequencies, activating the radio when the designated call sign is received. "The designated call sign" means the radio's own call sign.

There's a decent article about ALE on Wikipedia for those wanting more information.

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Biggest number wins, unsurprisingly, so the answer is 300 baud packet.

Each mode lists its baud rate (45, 31, 300), other than AMTOR which is 100 baud. And all four modes have 1 bit per symbol which means that is also the bit rate. More bits per second = faster data throughput.

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