Before backing with a helper, which hand signal is most important to agree upon?
Correct answer: D – Stop
Before backing, the driver and helper should agree on a set of hand signals they both understand, especially a clear stop signal. The helper should stand near the rear where the driver can see them. The driver should stop if the helper gives the signal or disappears from view.
Source: West Virginia Commercial Driver’s License Manual — § 2.2.4, “Backing Safely,” “Use a Helper”


