Final answer:
The relative horizontal disparity in projected depth is calculated by dividing the disparity by the viewing distance, option C.
Step-by-step explanation:
The question asks about the approximate formula for determining the relative horizontal disparity in projected depth. The disparity is the difference in the images in the two eyes that allows for depth perception, as stated in the provided information referencing retinal disparity and interocular distance. The correct formula for calculating the relative horizontal disparity in projected depth among the options given is C) Disparity divided by viewing distance. This is because the disparity, or the difference between two images perceived by each eye, is directly proportional to the depth perception when you factor in the viewing distance. Other factors such as convergence angle and interocular distance are not directly used in the approximation formula given in the options.