Which type of modulation alters the frequency of the carrier wave?

Frequency modulation is the correct choice because it specifically involves varying the frequency of the carrier wave to encode information. In this modulation technique, the frequency of the carrier wave increases and decreases based on the instantaneous amplitude of the input signal, which results in a change in the overall frequency of the transmitted signal.

This method is widely used in various types of communication systems, such as FM radio broadcasting. The advantage of frequency modulation is that it is generally more resistant to signal degradation caused by noise and interference compared to amplitude modulation. As a result, frequency modulation helps improve the clarity and fidelity of the transmitted signal, making it ideal for audio and music transmission.

Other modulation types listed either manipulate different aspects of the carrier wave or focus on varying the amplitude rather than frequency. For instance, amplitude modulation alters the strength or amplitude of the carrier signal without changing its frequency. Similarly, phase modulation changes the phase of the carrier wave rather than its frequency. Envelope modulation, while it influences the overall shape of the wave envelope, does not directly deal with altering the core frequency of the carrier. Thus, frequency modulation stands out as the method that distinctly modifies the carrier wave's frequency.