The correct option is A Condom
Barrier methods like condoms, made of thin rubber or latex sheath are used to cover the penis of a male and vagina or cervix in case of females.
This prevents the meeting of sperm with ovum and thus prevents pregnancy. In recent years, the popularity of condoms has increased due to its additional benefits of protecting the user from contracting STDs like AIDS, syphilis and gonorrhea, etc. This is because of the prevention of pathogen-containing fluid getting introduced into the partner during the intercourse.
In the other methods given in the options, there will be introduction of semen into the partner. Hence they are ineffective in preventing the transmission of STDs.