The statement that must be true about the two postal delivery workers is: They each have a displacement of 0 miles.
Displacement is defined as the change in position from the starting point to the ending point. Since both workers start and end their routes at the post office, their displacement is 0 miles, regardless of the distance they traveled along their routes. The other statements do not necessarily hold true for both workers, as they could travel different distances, have different velocities, and so forth.