What term describes the progressive silvering of a horse's coat?

The term that describes the progressive silvering of a horse's coat is "gray." When a horse is classified as gray, it signifies that the horse's coat color gradually changes over time as it ages. Gray horses are typically born with darker coats, which then lighten progressively to a silver or nearly white coloration as they mature. This process can vary in duration and intensity from one horse to another.

The other options represent different coat color patterns or types. A chestnut refers to a solid reddish-brown color without any silvery or gray characteristics. Roan describes a coat that is characterized by a mixture of colored and white hairs, but it does not indicate a progressive lightening effect like gray. Silver Dapple, while it refers to a specific color pattern where black horses have a mixture of silver-gray and black hairs, does not encompass the broader concept of the gradual silvering process that defines gray horses. Thus, "gray" accurately captures the essence of the progressive silvering observed in the coats of such horses.