After ovulation, the Graafian follicle becomes an endocrine organ called

The correct option is corpus luteum.

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

1. Understanding the Ovarian Cycle:
During the follicular phase of the ovarian cycle, ovarian follicles grow and mature under the influence of hormones. The fully mature follicle is known as the Graafian follicle, which contains the mature secondary oocyte.

2. The Process of Ovulation:
During ovulation, a surge in luteinizing hormone (LH) triggers the rupture of the Graafian follicle, releasing the mature oocyte into the fallopian tube (ovarian tube) where it can potentially be fertilized.

3. Formation of the Corpus Luteum:
After the release of the oocyte, the remnants of the ruptured Graafian follicle undergo a transformation. Under the continued influence of LH, the granulosa and theca cells of the follicle proliferate and accumulate lipids, transforming into a temporary endocrine structure called the corpus luteum (which means "yellow body" in Latin).

4. Function as an Endocrine Organ:
The corpus luteum acts as a vital endocrine gland. It primarily secretes large amounts of progesterone, along with some estrogen, to prepare and maintain the lining of the uterus (endometrium) for a potential pregnancy.

5. Reviewing the Other Options:
- Fibrin: This is a fibrous, non-globular protein involved in the clotting of blood, unrelated to ovarian structures.
- Ovarian tube: Also known as the fallopian tube or oviduct, this is the anatomical duct through which the egg travels, not an endocrine structure formed from the follicle.
- Globulin: This is a family of globular proteins that play important roles in the immune system and liver function, not an endocrine organ.