Answer:
D. Reins are leather straps that attach the horse's mouth to the rider's hands to help guide the horse.
Step-by-step explanation:
Mia's essay will be about the equipment needed for horseback riding. Our task is to determine which statement is a supporting detail - what piece of information is related to the essay topic, additionally expanding, illustrating, describing, explaining, or clarifying it.
The correct option is option D. Reins are a very important piece of equipment, and including information about it is something Mia would do.
The rest of the statements are not related to equipment. The first sentence tells about Mia's personal experience, the second tells about the cost of horse maintenance, and the third tells about safety while horseback riding. They are related to different topics, which is why they are incorrect.