(A) Wow. They packed plenty of errors into this choice.

- less ... as should be less ... than.

- with comparable size should be of comparable size. They are not with a size.

- to be contributing does not really make sense. They have been determined to contribute.

(B) less ... as should be less ... than.

- they have determined to contribute distorts the meaning of the sentence, conveying that the engines have determined to contribute gases to global warming.

(C) This one correctly uses less ... than, and the rest of it make sense.

(D) Diesel engines burn ... ,and emitting ... is not parallel.

- which have been determined as contributors distorts the meaning, seeming to convey that as contributors, the gases have been determined.

(E) This one conceivably conveys that the diesel engines burn less fuel than they burn gasoline engines.

- whose size is comparable conveys that the size is comparable. That's nice. The size is comparable, to what though?

- determined as contributing is not idiomatically correct. It should be determined to contribute.

The correct answer is C.