Final answer:
The most commonly used barrier method of contraception is the male condom, which is effective in preventing both pregnancy and STIs.
Step-by-step explanation:
The most widely used barrier method of contraception is the a) male condom. Barrier methods of contraception involve devices that physically block sperm from entering the uterus. The male condom is not only the most commonly used barrier method but also a prevalent form of birth control globally due to its affordability, accessibility, and effectiveness in preventing both pregnancy and sexually transmitted infections (STIs). Other barrier methods include the female condom, cervical cap, diaphragm, and sponges that may contain spermicides to increase effectiveness.