Final answer:
The most common vocalization between separated mares and foals is the whinny. It is a loud and long-distance communication sound that horses use to locate each other when apart.
Step-by-step explanation:
The most common vocalization between mares and foals who are separated is the: A) Whinny.
Horses have a variety of vocalizations they use to communicate with each other, and the whinny is a well-known sound that horses make. This vocalization can carry over long distances and is often used by horses when they are attempting to locate each other or when separated, such as between a mare and her foal. The description in the reference text provides an example of horses expressing their excitement and looking for their mistress with whinnies, which supports the idea that whinnies are used in such situations.