Which one is not a barrier method for birth control?

The correct answer is Option (4) → Intra Uterine Devices (IUDs)

A barrier method physically stops the ovum and sperms from meeting.

Option (1) Diaphragms — A barrier method. Diaphragms are rubber barriers inserted into the female reproductive tract to cover the cervix during coitus; they block the entry of sperms through the cervix.

Option (2) Cervical caps — A barrier method. Cervical caps, like diaphragms, are rubber barriers placed over the cervix during coitus so that sperms cannot enter.

Option (3) Vaults — A barrier method. Vaults are also rubber barriers that cover the cervix during coitus and prevent conception by blocking the sperms.

Option (4) Intra Uterine Devices (IUDs) — Not a barrier method. IUDs are placed inside the uterus; they increase phagocytosis of sperms and copper ions reduce sperm motility, but they do not block the way.

NCERT Extract:
Population Stabilisation and Birth Control – “… In barrier methods, ovum and sperms are prevented from physically meeting with the help of barriers.”
Population Stabilisation and Birth Control – “… Diaphragms, cervical caps and vaults are also barriers made of rubber that are inserted into the female reproductive tract to cover the cervix during coitus. They prevent conception by blocking the entry of sperms through the cervix.”