Final answer:
A red light on an Ethernet interface usually means that the Ethernet cable is disconnected. Other issues might include cable damage or port issues, but a disconnected cable is the most direct interpretation of this signal.
Step-by-step explanation:
If an Ethernet interface in the Network System Preference Pane shows a red light, this generally signifies that the Ethernet cable is disconnected (Option A). This color coding is used in many operating systems to quickly convey the status of the network connection. A red light typically means there is no physical link between your computer and the network device, such as a router or switch, due to reasons like a disconnected cable, faulty cable, or a problem with the network port. It does not indicate that the interface is operating at maximum speed or that it is experiencing a hardware malfunction, which excludes options C and D. While it could suggest a failure to receive data (Option B), the most common and direct meaning is simply that there is no active physical connection.
To resolve this issue, you should check if the Ethernet cable is indeed disconnected and, if so, properly reconnect it. If problems persist after reconnecting the cable, further troubleshooting would include checking for cable damage, testing different ports on the router, or considering the possibility of a malfunctioning network interface card (NIC). In some cases, diagnosing a hardware malfunction may require expert analysis and potential replacement of faulty components.