Final answer:
The correct configuration to ensure a switch is the root switch in STP is option C - spanning-tree vlan 10 root primary.
Step-by-step explanation:
The correct configuration to ensure that a switch always becomes the root switch in the Spanning Tree Protocol (STP) is option C: spanning-tree vlan 10 root primary. This configuration sets the switch as the root switch for the specified VLAN. Option A (spanning-tree vlan 10 priority 0) and option B (spanning-tree vlan 10 priority 61440) do not guarantee that the switch will be the root, as other switches may have even lower priority values. Option D (spanning-tree vlan 10 priority 4096) sets a higher priority for the switch, but it does not guarantee that it will be the root if other switches have even lower priority values.