Atretic follicles occur in

The correct option is Ovary.

Here is the step-by-step educational explanation of why this is correct:

1. Understanding Atresia:
Follicular atresia is the periodic, hormonally controlled breakdown of immature ovarian follicles. An ovarian follicle is a rough spheroid cellular aggregation set found in the ovaries that secretes hormones and contains an oocyte (immature egg) that may eventually be released during ovulation.

2. What are Atretic Follicles?
During every menstrual or estrous cycle, several follicles begin to develop. However, usually only one follicle (known as the dominant follicle) reaches maturity to release an egg during ovulation. The remaining follicles that started the process but failed to mature undergo a degenerative process called atresia. These degenerating, non-dominant follicles are referred to as atretic follicles.

3. Where does this occur?
Since follicles and oocytes are fundamental structural units of the female reproductive system responsible for producing eggs and female sex hormones, follicular development and atresia occur exclusively in the female gonads, which are the ovaries.

Therefore, atretic follicles occur specifically in the Ovary, and not in the liver, testis, or thymus.