The method of contraception which help in preventing transmission of sexually transmitted diseases is

Concept:

• The devices that are used to prevent unwanted pregnancies are called Birth control devices or Contraceptives.
• Various Contraceptive methods are:

1. Natural/traditional methods
2. Barrier methods - Condoms, Diaphragms, Cervical caps
3. Intrauterine devices (IUDs) - Lippes loop, Copper T, Copper 7
4. Oral contraceptives - Saheli
5. Injectables and Implants 
6. Surgical Methods - Vasectomy & Tubectomy

Explanation:

• Condoms are barriers made of thin rubber/latex sheath used to cover the penis in the male or vagina and cervix in females. It prevents the deposition of ejaculated semen into the vagina of the female.
• Condoms help in preventing the transmission of sexually transmitted diseases, as condoms act as a barrier between the direct contact of sexual organs during intercourse.