For which of the following modes is a Class C power stage appropriate for amplifying a modulated signal?
Why A Class C stage conducts for less than half of each RF cycle, so its output is a series of current pulses that a tuned tank reshapes into a sine wave. That process is highly nonlinear and destroys any amplitude information in the drive signal, but it is very efficient. FM carries its information in frequency alone at constant amplitude, so nothing is lost and Class C works fine.
Watch out SSB and AM both vary the envelope amplitude, so a Class C amplifier would clip and distort them, generating splatter. Those modes need a linear stage, typically Class A or Class AB.
Class C kills amplitude. Constant-envelope modes only: FM and CW yes, SSB and AM no.