Final answer:
The stage of the estrous cycle in which an animal is receptive to mounting is Estrus. This phase is closely tied to ovulation, ensuring the presence of fertilizable ova. Species like the pronghorn antelope experience a seasonal estrus while primates can engage in reproductive behavior at nearly any time.
Step-by-step explanation:
The stage of the estrous cycle in which an animal stands to be mounted is Estrus (B). During this phase, the female is sexually receptive, which is also often referred to as being 'in heat'. This is a part of the reproductive cycle where the female displays behaviors indicating she is ready for mating, primarily because this phase coincides with ovulation, therefore ensuring that there are fertilizable ova within the female genital tract.
Iteroparity is a characteristic of species that reproduce multiple times over the course of their lives, and some animals have a seasonal estrus cycle when they go 'into heat'. Unlike these species, humans and other primates can engage in reproductive behaviors at nearly any time during the reproductive years, despite having a menstrual cycle with a limited fertility window each month.
In comparison, female sheep have an estrus cycle that is influenced by daylight length and typically doesn't commence until the days start to get shorter. Understanding these cycles is important for managing breeding in livestock and in studying animal behavior and physiology.