The full form of SIM is

Subscriber's Identity Module
Subscriber's Identity Machine
Self Identity Machine
Self Identity Module

The correct answer is (a) Subscriber’s Identity Module.

A SIM card is a small, thin piece of plastic that is used to identify a mobile phone user. It is typically inserted into the phone’s SIM slot and contains the user’s phone number, contact information, and other personal data. SIM cards are issued by mobile phone operators and are required to use a mobile phone network.

The other options are incorrect. (b) Subscriber’s Identity Machine is not a correct term. (c) Self Identity Machine is not a correct term. (d) Self Identity Module is not a correct term.