Final answer:
In horses, a deciduous premolar that fails to fall out is referred to as a retained deciduous premolar, similar to the replacement process of deciduous teeth with permanent teeth in humans.
Therefore, option A is correct.
Step-by-step explanation:
In horses, a deciduous premolar that fails to fall out is called a retained deciduous premolar. This condition is analogous to what may occur in human dental development, where baby teeth, or deciduous teeth, are normally lost and replaced by permanent teeth during middle childhood. It is essential that these retained deciduous premolars are dealt with properly, as they can cause dental issues for the horse.
During middle childhood in humans, the deciduous teeth are replaced by permanent teeth, which generally include the eight incisors first, followed by the eight premolars, and then the four canines. Additionally, the second permanent molars and wisdom teeth emerge later in life, often requiring surgical intervention if impacted.