Final answer:
The term 'maiden mare' is used to describe a female horse that has not been bred. The other options given, such as 'open,' 'wet,' and 'brood,' have different meanings in the context of horse breeding.
Step-by-step explanation:
A maiden mare is one that has never been bred. The correct answer to the question - A(n) ____ mare is one that has never been bred - is a) Maiden. By definition, a maiden mare is a female horse that has not been bred, meaning she has not had any foals. Other terms used in the context of breeding horses include 'open,' which refers to a mare that is not currently pregnant; 'wet,' describing a mare that is lactating and has a foal at side; and 'brood,' which refers to a mare used for breeding.