At a first glance (seeing the initial post, I agree with Tharsim.)
Then with the correction you have done. (I may be mistaken, but then, at least you can´t say I didn´t give it a try!)
An(A) engine is an equipment(B) that produces(C) power to make a
So, alternative D.
To my thinking, the correction would be as follows:
An engine is an equipment that produces power to make vehicles move.
Vehicles taken in general, so the engine wouldn´t be tought in specific way, but in general ways too.
I hope I had not confused you further.
However it´s a hunch of mine, I must let you be aware of that.
Ah, anyway if "a vehicle" is to remain in the sentence, it would take a plural "moves". Which get us to the letter (D) again.