Which depth cue can Brenda not use if she has only one eye?

The correct answer highlights the importance of retinal disparity as a depth cue that relies on having two eyes. Retinal disparity occurs because each of your eyes views an object from a slightly different angle; the brain then processes these two different images to gauge depth. This phenomenon is crucial for depth perception and is fundamentally a binocular cue, meaning it requires both eyes to function.

In contrast, the other options—relative size, interposition, and texture gradient—are all considered monocular depth cues. These cues can be utilized even when only one eye is available. Relative size helps the brain interpret the size of objects based on their distance; interposition involves understanding that closer objects obscure those further away; and texture gradient refers to the density of visual details as distance increases, providing depth perception even with a single eye. Thus, since Brenda has only one eye, she cannot make use of retinal disparity, making it the only depth cue she is unable to utilize.