Which command configures this switch to always be the Spanning Tree root for VLAN 750?

The command spanning-tree vlan 750 priority 0 is correct because a lower Spanning Tree Protocol (STP) priority value makes a switch more likely to be elected as the root bridge. The lowest possible priority value is 0, ensuring this switch will always be the root for VLAN 750, assuming no other switch also has a priority of 0 and a lower MAC address. spanning-tree vlan 750 root primary is incorrect because while it attempts to make the switch the root, it sets the priority to 24576 if no other switch has a lower priority, or to 4096 less than the current root's priority. This doesn't guarantee it will always be the root if a switch with a priority of 0 or a very low priority is introduced later. The other options, spanning-tree vlan 750 priority 38418607 and spanning-tree vlan 750 priority 614440, use high priority values, making the switch less likely to be the root bridge. STP priorities are configured in multiples of 4096, so 38418607 and 614440 are not valid priority values.