to inform workers on the proper use and function of machines
While mechanical comprehension does involve mechanisms and machines, this does not adequately answer the question; the question asks for the purpose, not components. Mechanical comprehension deals with a wide array of mechanisms and does not merely focus on the tools used by car mechanics, nor the training undergone by vehicle mechanics.
B to learn the use of various mechanisms
C to describe the training undergone by car mechanics
D to determine how to adequately use mechanic's tools