Step 1: Progestogen-only pills (POP, mini-pills) contain only a synthetic progestin and no estrogen, which makes them safe during breastfeeding.
Step 2: Their contraceptive action is multi-pronged. They thicken the cervical mucus so sperm penetration toward the egg becomes difficult.
Step 3: They thin and render the endometrial lining hostile, preventing implantation of a fertilised egg.
Step 4: They also inhibit ovulation in a proportion of cycles. Since options a, b, and c are each correct mechanisms, the complete answer is all of the above.