Final answer:
The physiological change occurring when the muscles relax in the eye is that the lens flattens for distant vision due to relaxation of the ciliary muscles. This relaxation causes tension in the zonular fibers, leading to a less convex lens.
Step-by-step explanation:
When muscles relax in the eye, specifically the ciliary muscles, the physiological change that occurs is D. The lens flattens. This happens during the process of accommodation when the eye adjusts for viewing distant objects. The ciliary muscles relax, which causes tension in the zonular fibers, resulting in the lens becoming less convex, or flatter. When the muscles are contracted for near vision, the lens thickens to increase its refractive power. Conversely, for distant vision, relaxing these muscles allows the lens to flatten, which reduces its refractive power and acts to keep distant images in clear focus.
Additionally, the pupillary reflexes involve constriction and dilation. For example, under bright lighting conditions, the parasympathetic postsynaptic neuron releases acetylcholine, which stimulates the circular iris muscles to contract, causing the pupil to constrict and limit the amount of light entering the eye.