A ____ mare is one that has never been bred.

A mare described as a maiden is one that has never been bred. This term is specifically used in equine terminology to refer to a female horse that has not yet undergone the experience of breeding, making it clear that she has not had any foals or attempted reproduction.

In contrast, other terms present in the options refer to different reproductive statuses in mares. An open mare is one that has been bred in the past but is currently not pregnant; a barren mare indicates that she has attempted breeding but has not conceived; and a foaling mare is one that is in the process of giving birth or has recently given birth. These distinctions are important in horse breeding and evaluation, as they indicate the reproductive history and potential of the mare in question.