Which choice is a NID placed at the demarcation point between an ISP and customer's networks that can be used as a troubleshooting tool when diagnosing a network issue?
Answer Description
A smart jack is a Network Interface Device (NID) placed between two networks to provide diagnostic data. Typically used by Internet Service Providers (ISP) to help troubleshoot issues on a customer's network.
Wikipedia
In telecommunications, a network interface device (NID; also known by several other names) is a device that serves as the demarcation point between the carrier's local loop and the customer's premises wiring. Outdoor telephone NIDs also provide the subscriber with access to the station wiring and serve as a convenient test point for verification of loop integrity and of the subscriber’s inside …
Network interface device - Wikipedia, the free encyclopedia