Explanation:

In computer networking, error-reporting messages are used to inform the source of a network packet about any issues encountered during the transmission or delivery of the packet. These messages are generated by various networking devices and protocols to provide feedback and help troubleshoot network connectivity problems.

The given options are:

a) Destination unreachable: This is a type of error-reporting message that is sent by a router or a network device to inform the source that the destination host or network is unreachable. This can occur due to various reasons such as a network congestion, a misconfigured router, or a firewall blocking the packet.

b) Source quench: This is another type of error-reporting message that is used by a router to inform the source that the network is congested and it should reduce the rate at which it is sending packets. This helps in preventing network congestion and improves the overall network performance.

c) Router error: This option is incorrect as it is not a specific type of error-reporting message. A router can generate various types of error messages depending on the situation, but "Router error" is not a standard category of error messages.

d) Time exceeded: This is a type of error-reporting message that is sent by a router when the time-to-live (TTL) value of a packet reaches zero. The TTL value is decremented by each router that forwards the packet, and if it reaches zero, the router generates a time exceeded message and sends it back to the source. This helps in detecting routing loops or packets that are stuck in the network.

Conclusion:

In summary, the correct answer is option 'C' - Router error, as it is not a specific type of error-reporting message. The other options, destination unreachable, source quench, and time exceeded, are all valid types of error-reporting messages used in computer networking.