Which command enables Layer 3 routing on a multilayer switch?

The command that enables Layer 3 routing on a multilayer switch is "ip routing." This command is essential because it activates the routing capability on the switch, allowing it to forward packets based on their IP addresses instead of simply switching frames at Layer 2. When "ip routing" is configured, the multilayer switch can participate in routing protocols, make route decisions, and facilitate communication between different VLANs. This capability is crucial for environments that require inter-VLAN routing or connectivity between different subnets.

The other choices are not valid commands for enabling routing. "ip access" does not exist as a command in IOS to activate routing; instead, it's part of configuring access control lists (ACLs). "ip forwarding" is also not a command used for enabling routing; it might refer to packet forwarding in a more general sense but does not explicitly configure Layer 3 routing functionality on a device. "enable routing" is not a standard IOS command, and there isn’t a command by that name for configuring Layer 3 functionalities. Thus, "ip routing" is definitively the correct command for this purpose.