What part of the body supports the weight of the saddle and rider?

The back of the horse is the primary structure that supports the weight of both the saddle and the rider. This area is designed to bear weight efficiently due to its broad and muscular structure, which provides the necessary strength and stability. When a horse is being ridden, the weight is distributed across the back, allowing for a balanced riding experience.

The withers, flank, and croup also play important roles in the horse's anatomy but are not as directly responsible for supporting the saddle and rider's weight. The withers serve as a point of contact for the saddle and help keep it in place, while the flank is primarily associated with movement and flexibility, and the croup is involved in propulsion and balance. However, the back is where the main support occurs, making it the correct choice for this question.