A server will not power on. Which of the following devices should the administrator use FIRST to determine the problem?

Loop back plugs are used to test network interface cards or other ports for connectivity and functionality, which is irrelevant if the server doesn't power on.

If a server will not power on, the power supply is a common component to fail, and a power supply tester provides immediate feedback on whether the PSU is outputting the correct voltages. This tool quickly isolates whether the issue is with the power unit itself, which is critical for initial power-on problems.

System logs are stored on the server's hard drive and require the system to at least partially boot to be accessed, which is not possible if the server won't power on.

A TDR (Time-Domain Reflectometer) is used to locate faults in metallic cables (e.g., network, coaxial) by measuring signal reflections, and it is not relevant for a server that fails to power on.