What is required to modulate a signal?

Modulating a signal involves varying one or more of its key characteristics, which include amplitude, frequency, and phase. In the context of modulation, it's important to understand that you do not have to change all three parameters at once; you can modulate the signal effectively by varying just one of these parameters.

For example, in amplitude modulation (AM), the amplitude of the carrier signal is varied, while the frequency and phase remain constant. In frequency modulation (FM), it is the frequency of the carrier signal that varies while amplitude stays the same. Phase modulation changes the phase of the carrier signal, again leaving amplitude and frequency unchanged. Therefore, the correct response reflects the concept that modulation can be accomplished by varying one of these three key signal parameters, making it a versatile technique in communications.