By keying an FM transmitter with a sub-audible tone
By changing an oscillator’s frequency directly with a digital control signal✓Правильный ответ
By using a transceiver’s computer data interface protocol to change frequencies
By reconfiguring the CW keying input to act as a tone generator
Объяснение
Direct FSK shifts the carrier oscillator itself between two frequencies under control of the data bits, with no audio stage in between. Keying an FM transmitter with tones is audio frequency shift keying, which produces a similar on-air signal but is indirect rather than direct.