A) Estrogen secretion further decreases- Estrogen is secreted by the corpus luteum. Since it disintegrates in the absence of fertilization, estrogen levels fall.
B) Progesterone secretion starts declining- If pregnancy occurs, human chorionic gonadotropin is released maintaining the corpus luteum allowing it to maintain levels of progesterone. In the other way round, (i.e., in case of fertilization does not occur), progesterone levels fall due to the disintegration of the corpus luteum.
C) Corpus luteum will disintegrate- If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays (after approximately 14 days in humans). It then degenerates into a corpus Albicans, which is a mass of fibrous scar tissue.
D) Primary follicle starts developing- Primary follicle is a precursor to the corpus luteum and thus this is most unlikely to happen. At the start of the menstrual cycle, some 12-20 primary follicles begin to develop under the influence of elevated FSH to form secondary follicles. The primary follicles have formed from primordial follicles, which developed in the ovary at around 10-30 weeks after conception.
So, the correct answer is option B.