The correct answer is: C. loop backs (wrap plugs)
A loop back is a diagnostic tool used to test the functionality of a serial or parallel port. It does this by creating a loop between the transmit and receive pins of the port. This allows the port to send data to itself and verify that it is working properly.
A high volt probe is a tool used to measure high voltage signals. It is not typically used to test serial or parallel ports.
A cable scanner is a tool used to identify and troubleshoot problems with cables. It can be used to test serial and parallel ports, but it is not the most effective tool for this purpose.
A sniffer is a tool used to monitor network traffic. It can be used to test serial and parallel ports, but it is not the most effective tool for this purpose.
None of the above options are the most effective tool for testing serial and parallel ports. The most effective tool for this purpose is a loop back.