Which command configures the spanning-tree root for a specific VLAN?

The command that configures the spanning-tree root for a specific VLAN is indeed identified correctly as "spanning-tree vlan vlan-id root primary." This command sets the specified VLAN's root bridge to become the primary root bridge for the spanning tree.

Using "root primary" instructs the switch to adjust its bridge priority to a value that ensures it is elected as the root bridge for that VLAN, provided that no other configured switches have a priority lower than the default (which is usually 32768). This is particularly useful in larger networks where it’s essential to establish a designated root bridge to maintain efficient routing and prevent broadcast storms.

Other options presented either inaccurately describe the required syntax or misstate the nature of the command structure, making them unsuitable for configuring the spanning-tree root bridge as intended for a specific VLAN.