Final answer:
The follicle that stays behind and becomes the corpus luteum is the collapsed follicle that remains in the ovary after ovulation. It undergoes luteinization to transform into the corpus luteum, which produces progesterone and maintains the endometrium.
Step-by-step explanation:
The follicle that stays behind and becomes the corpus luteum is the collapsed follicle that remains in the ovary after the oocyte has been ovulated. This process is called luteinization. After ovulation, the granulosa and theca cells of the collapsed follicle undergo luteinization and transform into the corpus luteum, which starts producing large amounts of progesterone. The corpus luteum secretes progesterone, which maintains the endometrium and prepares it for pregnancy.