In what state is the animal during estrus?

An animal in the state of estrus is characterized by being receptive to breeding. During this phase of the estrous cycle, hormonal changes occur that prepare the animal for potential mating. Estrus signifies a peak in fertility, when the animal is physically and behaviorally prepared to accept a male for reproduction. This period is often marked by specific behaviors, such as increased vocalization, restlessness, and willingness to stand for mating, indicating the animal's readiness to mate.

The other choices do not accurately describe the estrus state. Active pregnancy refers to a stage when an animal is already pregnant and thus not in estrus. Not cycling indicates that the animal is either in a phase of the estrous cycle that is not receptive to mating or is experiencing anestrus, while nursing young suggests that the animal is engaged in caring for offspring and not in a breeding circumstance. Thus, the ideal answer is that during estrus, the animal is receptive to breeding.