The correct answer is b. hyracotherium.
Hyracotherium, also known as Eohippus, is considered the earliest known ancestor of the modern horse. Fossil evidence indicates that it was a small animal, standing only about 12 to 14 inches tall at the shoulder, and it had four toes on its front feet and three toes on its hind feet.
Over time, the horse evolved to become larger and faster, and its toes also underwent changes. Fossils of later horse ancestors such as Mesohippus, Merychippus, and Pliohippus show a gradual reduction in the number of toes until only a single toe remains. The modern horse has a single toe on each foot, which is supported by a single large bone, the equivalent of the middle toe in the human foot.
Therefore, according to the fossil record, Hyracotherium or Eohippus was the species in the horses' ancestry that was likely the first to run on three toes.