What letter appears next to a route created automatically by RouterOS when a valid address is added to an active interface?

The correct answer is that when a valid address is added to an active interface in RouterOS, the letter that appears next to the automatically created route is "C."

This designation stands for "connected." When RouterOS recognizes that an IP address has been assigned to an interface, it automatically generates a connected route for that address. This route allows devices within the same network to communicate effectively. The "C" prefix in the routing table indicates that this is a directly connected network and the route is based on the interface's IP configuration.

This is a standard behavior in many routing protocols and network configurations, providing a clear indication of route origins and types for network administrators reviewing routing tables.