Which of the following features in an audio/video (A/V) receiver is user-adjustable?

The feature that is user-adjustable in an audio/video (A/V) receiver is amplification gain. Amplification gain controls the volume level and the strength of the audio signal that is sent to the speakers. Users can adjust it according to their preference or to accommodate the listening environment, enhancing or reducing the audio output as needed.

Response delay in signal processing, although it may seem relevant, is not typically something users adjust on a regular basis; it is more related to the internal functioning of the A/V receiver to synchronize audio and video signals. Fixed channel output refers to a predetermined setup within the receiver, which does not allow for user alterations, and pre-set audio modes are generally functions designed by the manufacturer that provide specific sound profiles but may not allow significant user adjustment beyond simple selections. Thus, amplification gain is the most directly user-interactive aspect of an A/V receiver, enabling personalized sound experiences.