Which of the following designates a mature male cat?

A mature male cat is specifically referred to as a "Tom." This term is widely recognized in both casual and formal contexts to identify male felines once they reach sexual maturity. The nomenclature stems from traditional naming conventions used in animal husbandry and breeding.

The other terms listed do not apply to cats. A "Buck" is used to describe a mature male goat, a "Stallion" is designated for mature male horses, and a "Sow" refers to a female pig. Thus, understanding the terms associated with various animals helps in correctly identifying their genders, especially in the context of breeding and animal science.