Which of the following is a loopback IP address?

The correct option is A (127.0.0.1).

A loopback IP address is a special IP address used by a host to send network traffic to itself. It is primarily used for troubleshooting, testing network software locally, and verifying that the local network stack (TCP/IP stack) is functioning correctly without sending packets out onto the physical network.

• Reserved Range: The entire Class A block from 127.0.0.0 to 127.255.255.255 (or 127.0.0.0/8 in CIDR notation) is reserved for loopback functions.

• Standard Localhost: Among this block, 127.0.0.1 is the most commonly used loopback address and is usually mapped to the hostname localhost.