Final answer:
The most common vocalization between separated mares and foals is the whinny, a high-pitched sound used for communication over distances.
Step-by-step explanation:
The most common vocalization between mares and foals who are separated is the whinny. This sound is a way for the horses to communicate over distances and re-establish contact with each other. As we can infer from the reference text, when separated, the horses express their emotions through various sounds. The whinny is a distinctive, high-pitched sound that carries well and is typically used by horses to greet each other or signal their location, as observed when the mistress calls and the horses respond with whinnies. Other vocalizations like the nicker, neigh, or snort have different meanings and contexts. The nicker is usually a softer, lower sound made when greeting or showing affection, often heard when a mare and foal are close. A neigh is similar to a whinny and sometimes used interchangeably. It is a loud, continuous sound also used for communication. The snort is a sound associated with alarm or curiosity, not the reunion of mare and foal.