If a speed mismatch was configured between two Cisco routers' Ethernet interfaces, what would the status of this interface be?

Down/down is the correct answer.

Explanation:

The down/down is the status of the interface when the speed of the ethernet mismatches that is configuration between the Cisco routers. Generally, it is the way of saying by the Cisco is "the connection" or the following ports are available for the connection but do not have one of them then, check their systems or the cables connection.