Explanation: A loopback test in amateur radio verifies your station's transmit and receive functionality without relying on an external station. Sending yourself a test message, particularly in digital modes like Winlink or APRS, directly achieves this. Your station transmits the message, which is then routed (often through a local digipeater or gateway, or even an internal system loop) back to your own receiver, allowing you to confirm that your equipment can both send and successfully receive its own transmissions. Option A refers to a more general diagnostic. Option B, testing IP address 127.0.0.1, only verifies your computer's internal network stack, not your radio's RF transmit and receive capabilities. Option D involves an external Land Earth Station (LES) and a satellite link, which tests an external system rather than your local station in a closed loop.