Which of the following is a mucous membrane of a horse?

The gums of a horse are classified as mucous membranes because they are soft tissues that line the inside of the mouth and are involved in secretion and protection. Mucous membranes play crucial roles in protecting underlying tissues and facilitating various bodily functions, including the production of mucus to keep surfaces moist.

In the case of horses, the gums are important not just for oral health but also for assessing the overall health of the animal; they can indicate hydration levels and circulation status through their color and moisture.

The other options, while they're part of the horse's anatomy, do not qualify as mucous membranes. The skin is primarily a protective barrier made up of epithelial cells and is classified as a dermal layer rather than a mucous membrane. The hoof is a hard structure that protects the sensitive parts of the horse’s foot and does not have the characteristics of a mucous membrane. The frog, which is a part of the hoof and aids in cushioning and traction, also does not fall under the definition of mucous membranes. Thus, the gums stand out as the sole mucosal tissue among the options provided.