Which command would you enter to configure the STP cost for VLAN 10 on an interface?

To configure the Spanning Tree Protocol (STP) cost for VLAN 10 on an interface, the command "spanning-tree vlan 10 cost cost" is used. This command allows you to explicitly set the path cost associated with that VLAN on the specified interface. The cost is important because it influences the decision-making process for the root bridge and topology changes within the STP framework. When you adjust the cost, you can manipulate the STP topology to favor certain paths over others, which is crucial for optimizing network performance and redundancy.

The other commands listed serve different purposes within the STP configuration. The command regarding port priority adjusts the priority of the port within the STP process, which can affect the selection of the designated and root ports but does not change the cost itself. The root primary and root secondary commands are used to configure a switch as the primary or secondary root bridge and do not directly influence the STP cost on a per-VLAN basis. Therefore, the correct choice specifically addresses the requirement to configure the STP cost.