Final answer:
The primary purpose of using reins while horseback riding is to steer the horse, providing control over direction and pace.
Step-by-step explanation:
The primary purpose of using reins while horseback riding is A) To steer the horse. Reins are an essential part of a horse's bridle and are used by the rider to give directions to the horse, signal commands, and help control the animal's pace. While they are sometimes decorated, and could incidentally provide a bit of shade, these are not their intended purposes. The most important function is to steer, making it possible to guide the horse left, right, slow down, and stop.