Final answer:
A loopback IP address begins with the number 127, which is used by a computer to send traffic to itself, commonly using the address 127.0.0.1 for testing purposes.
Step-by-step explanation:
A loopback IP address begins with the number 127. Therefore, the correct option is: b. 127. The loopback address range is reserved for a device to send network packets to itself. In IPv4, the most commonly used loopback address is 127.0.0.1. This address allows network applications and services running on a device to test and communicate with themselves without the need for external network connectivity. The loopback address is a fundamental tool in network troubleshooting and testing, ensuring that network-related processes function correctly within the device's own environment. Options a, c, and d represent different address ranges, but none of them are associated with the loopback functionality; 127 is the specific range designated for this purpose.