In FSK modulation, (a) the frequency of a carrier signal is varied by a digital signal. (b) the frequency of a digital signal is varied by a carrier signal. (c) the phase of a carrier signal is varied...

In FSK modulation,
(a) the frequency of a carrier signal is varied by a digital signal.
(b) the frequency of a digital signal is varied by a carrier signal.
(c) the phase of a carrier signal is varied by a digital signal.
(d) the amplitude of a carrier signal is varied by a digital signal.