Which of the following terms describes a three to four-year-old horse?

The term that describes a three to four-year-old horse is "colt." In horse terminology, a colt specifically refers to a male horse that is under four years old. This age designation is important in understanding the developmental stages of a horse, as colts are typically still in their growing phase and may exhibit youthful characteristics.

A foal is a horse that is less than one year old, so it does not apply to a three to four-year-old. A stallion refers to an adult male horse that is capable of breeding, usually older than four years, which again doesn't fit the age range specified in the question. A yearling denotes a horse that is between one and two years old, so it also does not accurately describe an animal that is three to four years old. Therefore, "colt" is the appropriate term for a three to four-year-old horse, distinguishing it from other categories relevant to the age and gender of horses.