The best tool to identify the correct cable is C. Toner probe.

A toner probe is a device that can be used to locate and trace a specific cable within a network rack or a wall outlet. A toner probe consists of two parts: a tone generator and a tone detector. The tone generator is connected to one end of the cable and sends an audible signal through the cable. The tone detector is used to scan the other end of the cable and pick up the signal. The tone detector emits a sound or a light when it detects the signal, indicating the correct cable.

A toner probe can be useful for network technicians who need to locate an end user’s Cat 6 cable within a network rack that is poorly documented and has no cable management system. A Cat 6 cable is a type of twisted pair cable that can support high-speed data transmission over Ethernet networks. A network rack is a frame that holds network equipment, such as servers, switches, routers, or patch panels. A cable management system is a set of devices or methods that organize and secure the cables within a network rack .

The other options are not the best tools to identify the correct cable, because they have different purposes or limitations. A cable tester (option A) is a device that can be used to test the functionality and quality of a cable, such as its continuity, resistance, or interference. A cable tester can help diagnose and troubleshoot faulty cables, but it cannot locate or trace specific cables within a network rack. A network tap (option B) is a device that can be used to monitor and analyze the traffic and data that pass through a network. A network tap can help capture and inspect network packets, but it cannot locate or trace specific cables within a network rack. A loopback plug (option D) is a device that can be used to test the functionality and connectivity of a port or an interface. A loopback plug can help verify if a port or an interface is working properly, but it cannot locate or trace specific cables within a network rack.

For more information about toner probes and other network tools, you can refer to the following web search results: [What Is a Toner Probe?], [What Is Cat 6 Cable?], [What Is a Network Rack?], [What Is a Cable Tester?], [What Is a Network Tap?], [What Is a Loopback Plug?].