Final answer:
Horizontal relative disparity is a relative depth cue because it depends on variations in the distance between objects and the observer, affecting how much disparity is seen between the images captured by each eye.
Step-by-step explanation:
Horizontal relative disparity is only considered a relative depth cue because it varies with distance changes (Option C). This concept pertains to binocular disparity, which is the slight difference in the images captured by each eye because of their separate positions on the face. Our brains process these differences to perceive depth. However, horizontal disparity is relative as it changes with the distance of objects from the observer. Closer objects will have a more significant disparity than further ones, helping the brain gauge depth. This cue is critical in binocular vision, as we rely on both eyes to perceive the 3-dimensional structure of our environment.
On the contrary, it does not rely on monocular cues, which are visual indicators of depth requiring only one eye. Neither does it depend on binocular rivalry, which is when two different images presented to each eye lead to a competition in visual perception. Additionally, while vertical disparity also exists, it plays a lesser role in depth perception and is not the reason horizontal disparity serves as a relative cue.