A loopback adapter can be used to test a network interface card by routing output signals back into the system as input. This allows the technician to verify that the circuitry and software drivers responsible for transmitting and receiving data on the NIC are functioning correctly without the need to connect to an external network. Other options like a toner probe or a cable tester are used to test cables and would not be capable of testing a NIC in isolation. A punchdown tool is used to connect wires to patch panels or blocks and is not used for testing NIC functionality.