Which command is used to enable routing on a router that is configured as a switch?

The command used to enable routing on a router that is configured as a switch is "ip routing." This command activates the routing process on the device, allowing it to forward packets between different network segments. By default, many switches operate in Layer 2 mode, meaning they primarily switch traffic within a single local area network (LAN). However, when you want to allow the switch to also perform Layer 3 routing functions, you must enter the "ip routing" command in the global configuration mode. This enables the device to manage traffic between different IP subnets and allows it to utilize its routing table.

The other options do not fulfill the requirement of enabling routing. For instance, the command that sets the switch to trunk mode is specifically for Layer 2 configurations and does not affect routing capabilities. Similarly, "router on" and "enable routing" are not standard commands in Cisco IOS for activating the routing feature, which means they won't achieve the intended purpose of enabling routing on the device. Thus, the correct command to perform this action is indeed "ip routing."