Why do we perceive depth with two eyes?

Answer: The brain compares the slightly different retinal images from each eye for depth.

The brain adds the separate distance signals from each eye together for depth. — Wrong. This option treats each eye as sending a separate distance reading that the brain totals. The eyes are set apart, so their views differ slightly, but the brain does not sum two distances. It compares the two retinal images, and the pattern of differences carries the depth information.

The brain reads the inward turning angle of both eyes as a depth cue. — Wrong. This option mistakes a secondary cue for the main one: the eyes do turn inward a little when looking at close objects, and the brain uses that angle. But convergence mainly helps at short range; it is not the main basis for depth with two eyes. The main cue is comparing the slightly different retinal images from each eye.

The brain compares the slightly different retinal images from each eye for depth. ✓ — Correct! The eyes sit slightly apart, so each eye gets a slightly different view of the same scene. The brain compares these retinal images, and the differences help reveal which objects are nearer or farther. This is binocular disparity, the main cue for stereo depth, though one eye still uses cues such as motion, size, and occlusion.