In TBS communications, what does frequency shift keying (FSK) involve?

Frequency Shift Keying (FSK) involves modulating the frequency of a signal to represent data. In this method of digital communication, different frequency signals correspond to different data values. For instance, one frequency may represent a binary '1', while another frequency represents a binary '0'. This modulation technique allows for the transmission of digital information over communication channels effectively and is particularly useful in applications such as digital data transmission, radio communications, and telecommunications.

The other choices describe different types of modulation. Varying the amplitude of a signal refers to Amplitude Modulation (AM), while changing the phase of a signal is characteristic of Phase Shift Keying (PSK). Since the question specifically asks about frequency shift keying, the modulation of frequency is the defining feature, making it the correct answer.