comparison

I say D. This is a tough one so i really dunno. But thats what I came up w/ in 1min.

E for me

guys OA is C

can you share the source of this question please, thanks in advance!

'its weight, only 8.1 grams, is far less than four quarters, which weigh 5.67 grams each.' --> is incorrect because the weight of the dollar coin is compared to four quarters. The correct sentence should compare how much the dollar coin weighs with how much four quarters weigh. Eliminate A.

That cleared, consider the following construction:
X will be used more as a substitute for Y than Z. --> this is ambiguous; it could mean a) x will be used more as a substitute for Y than z will be used as a substitute for Y or b) x will be used more as a substitute for y than as a substitute for z. To fix the ambiguity, if we mean the former we should say 'than will z' & if we mean the latter we should say 'than for z'.

That said, we can eliminate B and D.
The easiest problem to spot in E is the use of 'it' in 'is far less than it is for'. What is 'it' referring to? Very unclear. Eliminate.

That leaves us with C, which I think you have typed incorrectly. The correct version should read:

as a substitute for four quarters more than for the dollar bill because it weighs only 8.1 grams, far less than