Which among the following is a visual effect and not an audio effect and must be applied to a video layer?

A visual effect that must be applied to a video layer is a "filter." Filters alter the appearance of the video, such as adjusting colors, adding blur, or applying stylistic effects. In contrast, audio effects, like reverb or equalization, modify sound rather than visuals. Therefore, filters and other visual effects can transform the aesthetic of a video but do not impact the audio track.