Video Explanation

Text Explanation

This sentence focuses on a unique idiom.  This idiom is used for comparisons that describe how two quantities change --- both getting bigger together, or both getting smaller, or one bigger & one smaller, etc.  The structure of this idiom is

the [comparative adjective][clause #1], the [comparative adjective][clause #2]

Contrary to what one might expect, this constitutes a full independent clause by itself, and does not constitute a run-on sentence.  Also, the words "the" are crucial to this idiom.  It's a very strange idiom, almost certainly one that will confuse non-native speakers.  Fortunately, its structure has an almost mathematical regularity, which makes it easy to recognize and easy to construct. 

In this sentence, what are the two comparative adjectives?  The first one is "faster", a single word, and the second, slightly longer, is "less fuel efficient."  The general format therefore should be

the faster ……, the less fuel efficient …..

After each comparative, we should have a full
[noun] + [verb] clause.  Choice (C) is the only one that follows this
pattern.  All the others --- the
"if", the "when", the "that much less fuel
efficient" --- all of these variants are absolutely incorrect.  This idiom has a very precise structure, and
only choice (C) reproduces this
structure correctly in both halves of the sentence.  Choice (C)
is the only possible answer.