What is the effect of Progesterone-only pills?: (A) Completely suppresses ovulation, (B) Thin lining of uterus, (C) Thick cervical mucus, (D) All of the above
Answer is D. Ref. KDT. Page. 343
Progestogen-only pills have multiple biological effects on the female body.
Multitude effect of POP:

Inhibition of follicular development
Inhibition of ovulation
Suppression of corpus luteum formation
Changes in the nature of the cervical mucus which inhibits the passage of sperms into the uterine cavity – Thickening of cervical mucous secretion
Effects on the endometrium which affect implantation – by making it thin