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

Answer: Option D

One word substitute is Commute.

Travel: make a journey, typically of some length.
Run: move at a speed faster than a walk, never having both or all the feet on the ground at the same time.
Shuttle: a form of transport that travels regularly between two places.
Commute: travel some distance between one's home and place of work on a regular basis.