The bus has to go back and forth every six hours.

The correct answer is C. Shuttle.

A shuttle is a vehicle that travels back and forth between two or more places, especially on a regular schedule. In this case, the bus is a shuttle that travels back and forth between two points every six hours.

The other options are not as accurate. Travel is a general term for moving from one place to another. Run is a verb that means to move at a speed faster than a walk. Commute is a verb that means to travel regularly between one’s home and one’s place of work.