The correct option is C prevent fertilisation
The main purpose of contraception is that it not only helps couples to avoid unwanted pregnancies, hence curbing population explosion, but also helps couples to plan their family. There is a wide range of contraceptive methods available like- barrier methods, Intrauterine devices, oral contraceptives, implants, sterilisation or the natural methods.
The natural method of birth control mainly works on the principle of avoiding chances of sperms meeting the ovum. If the sperm and ova are not meeting inside the female reproductive organs, there is no chance of fertilisation and hence no implantation and pregnancies.