The full form of STD can be:

Standard OR
Subscriber Trunk Dialling OR
Sexually Transmitted Diseases
S.T.D : Subscriber trunk dialling (STD) (also known as subscriber toll dialling) is an term for allowing subscribers to dial trunk calls without operator assistance. This term however, is still widely prevalent in India to describe any national call made other than one's local unit.In India the term still applies and to dial out of one's unit area, the relevant city code has to be prefixed to the local number. This prefix starts with a 0. Certain numbers are also prefixed with 95 which are local calls within 500 km. .....!!!! Plz plz plz mark it as brainliest answer plz.....