Which term best describes a horse that is castrated?

A horse that is castrated is referred to as a gelding. This term specifically denotes a male horse that has undergone the surgical procedure of castration, resulting in the removal of the testicles. Geldings are typically more docile and easier to handle compared to their intact counterparts, making them popular choices for riding and other equestrian activities.

The other terms provided have different meanings: a colt refers to a young male horse that has not yet been castrated, while a stallion is an intact male horse that is capable of breeding. A yearling is a horse that is between the ages of one and two years, regardless of its sex or reproductive status. Therefore, the correct identification of a castrated male horse as a gelding is clear and distinct from these other classifications.