A simple path to choosing among the possible answers listed is to check that the resulting units are those of length. You may wish to review the readings for sub-subunit 3.1.3.
Attribution:
Stephanie Redfern, Ranjeet (Ron) Agarwala, and Dr. Steve Gibbs. Mechanics I. The Saylor Academy 2014, http://www.saylor.org/courses/me102/