Final answer:
The main advantage of oral contraceptives is their high degree of effectiveness, with a low failure rate when used correctly and the benefit of regulating menstruation.
Step-by-step explanation:
The main advantage of oral contraceptives is the high degree of effectiveness. This form of birth control has a failure rate of about one percent or less with perfect use, and approximately ten percent with typical use, due to challenges such as remembering to take the pill at the same time every day. Its effectiveness is owed to the prevention of ovulation and the thickening of cervical mucus to deter sperm penetration. Furthermore, the pill induces changes in the uterus lining, making it inhospitable for a fertilized egg. Birth control pills come in different forms, such as the combined pill with both estrogen and progesterone, which may also regularize and lighten periods but has an associated risk of blood clots, or the progesterone-only pill which does not carry the clotting risk.
Answer: 3) high degree of effectiveness