In an FM transmitter system, the amount of deviation from the centre frequency is determined solely by the:

In an FM transmitter, the modulating signal controls the behaviour of the carrier wave in two distinct ways. The key to understanding FM is separating the 'amount' of deviation from the 'rate' of deviation. The amount (or width) of frequency deviation from the centre frequency is determined solely by the amplitude (volume or strength) of the modulating signal. A louder audio signal pushes the carrier frequency further away from the centre. Conversely, the frequency (pitch) of the modulating signal determines how fast the carrier sweeps back and forth, not how far it goes. Therefore, options suggesting the modulating frequency determines the amount of deviation are incorrect.