The correct option is D Reins
Option C: Reins are straps attached to a horse and are used for guiding or directing a horse while riding. The given sentence also talks about controlling a horse. Hence the word 'reins' is most suitable. You pull at the reins of a horse to guide him where to ride. Hence Option C is correct.
Option A: Chains are occasionally used to control horses in order to keep them still in a certain place. They aren't used while riding, and a horse cannot be 'controlled by pulling at the chains'. Hence A is incorrect.
Option B: Similarly, 'ropes' are used to tie horses around a pole or a tree to keep them from running away. They aren't used by the rider while he is riding the horse. Hence B is incorrect.
Option D: The hair or mane of the horse is a part of the horse, it does not sound human to pull the horse's hair. Hence D is incorrect.