Corpus luteum starts to regress in

Answer: Option D

Corpus luteum starts to regress in Diestrus.
In the absence of fertilization, the corpus luteum will regress over time. A corpus luteum develops each time a woman ovulates so that a woman will produce a corpus luteum numerous times throughout her lifetime.
During diestrus, the ovaries show increased numbers of large follicles with a single, follicular fluid-filled antral cavity. Increased numbers of atretic follicles are also seen during diestrus.