Final answer:
The cue illustrated by the distant wave crests appearing smaller and closer together is 'A) Texture gradient', a monocular depth cue that indicates depth by the changing density and fineness of a surface's texture with distance.
Step-by-step explanation:
When you are standing on the beach observing the waves, you're encountering various depth cues that help you perceive distance. The phenomenon where the crests of distant waves appear smaller and closer together is illustrative of the texture gradient cue. Texture gradient is a monocular cue - it requires the use of only one eye - and it pertains to the perception that a surface's texture becomes denser and finer as distance from the observer increases. This cue provides information about the depth and distance of surfaces. Therefore, the correct answer to the question is A) Texture gradient. Unlike motion parallax which involves the relative speed of objects moving across the retina, or linear perspective which refers to parallel lines converging with distance, texture gradient focuses on the variation in surface detail to convey depth.