Which command is used to enable routing on an interface in Junos OS?

The command "set interface family inet" is the appropriate choice for enabling routing on an interface in Junos OS. By using this command, you are configuring the interface to handle IPv4 traffic, which is essential for routing capabilities. In Junos OS, each interface must be assigned a family that defines how it will process traffic. When you specify "family inet," you are indicating that the interface should be configured for Internet Protocol v4, thereby enabling it for routing functions.

Routing cannot happen on an interface that is not configured with a specific protocol family. Hence, specifying the family not only activates the interface for IP traffic but also allows the routing protocols to operate over that interface, making it an integral part of the routing process on the device.

Other commands listed do not serve this purpose: enabling an interface or specifying it as a routing interface doesn't correctly apply to the context of enabling routing on that interface. Thus, "set interface family inet" is the clear command needed to enable routing effectively.