Explanation: Frequency-Shift Keying (FSK) is a digital modulation technique where information is transmitted by changing the frequency of a carrier wave. When the "key" (representing the data signal, like a "mark" or "space") is depressed or changes state, the transmitter's output carrier frequency is shifted between two or more discrete frequencies. This shift in the output frequency encodes the data being sent. * **A) The shifting of frequencies by modulating the carrier antenna:** Incorrect. Modulation, including frequency shifting, occurs within the transmitter circuitry before the signal reaches the antenna. The antenna radiates the modulated signal but does not perform the modulation itself. * **B) Changing the frequency of the input when the key is depressed:** Incorrect. The input signal itself is not frequency-shifted; rather, its state (e.g., on/off) determines how the transmitter's *output* carrier frequency is shifted. * **C) Changing the frequency of the output when the key is depressed:** Correct. This accurately describes FSK: the state of the key (input information) directly causes the transmitter's output carrier frequency to shift, thereby conveying the information.