Which of the following depth cues requires the use of two eyes?

A) motion parallax
B) relative size
C) optic flow
D) binocular disparity

Binocular disparity is the only depth cue on this list that requires the use of two eyes. This depth cue is based on the difference in the images that the two eyes see, which allows us to perceive depth and 3D information about the environment. The other cues listed (motion parallax, relative size, and optic flow) can be perceived with just one eye.