What modulation type keeps the amplitude constant while changing the frequency?

The correct answer is based on the principle of how information is encoded in frequency modulation. In frequency modulation, the amplitude of the carrier wave remains constant while the frequency varies according to the amplitude of the input signal. This results in changes to the pitch of the transmitted sound without affecting its loudness.

Frequency modulation is particularly robust against noise and interference, making it a preferred method for high-fidelity transmissions such as FM radio and digital communications. By maintaining a constant amplitude, frequency modulation effectively enhances signal clarity and resilience, as distortions affecting amplitude do not impact the conveyed information.

In contrast, amplitude modulation changes the amplitude of the signal to encode information, thus resulting in variations that can be susceptible to noise. Pulse modulation involves transmitting information as a series of pulses, with characteristics like width or spacing varying to convey data. Phase modulation encodes information by shifting the phase of the carrier signal, and while it also alters a different aspect of the wave, it does not maintain constant amplitude as frequency modulation does.

These differences illustrate why frequency modulation is the correct answer, emphasizing its unique property of maintaining amplitude while varying frequency.