The statement that must be true about the two postal delivery workers is:
They each have a displacement of 0 miles.
Displacement refers to the change in position from the starting point to the ending point. Since both workers return to the post office after completing their routes, their displacement is 0 miles, regardless of the distance they traveled or the routes they took.
The other options regarding traveling the same distance, having the same velocity, or traveling a distance of 0 miles are not necessarily true.