उत्तर : 3. "टॉप लेवल डोमेन"
व्याख्या :

1. Extensions are called top-level domains.

2. File extensions are used so that the computer's operating system or OS can recognize the file type.

3. A file extension primarily refers to a suffix added to the end of a computer file (an object containing information, data or commands) to define the file type.