Officials at the Unites States Mint believe that the

Officials at the Unites States Mint believe that the Sacagawea dollar coin will be used more as a substitue for four quaters rather than for the dollar bill because its weight, only 8.1gms, is far less than four quaters, which weigh 5.67 gms each.

a) more as a substitue for four quaters rather than for the dollar bill because its weight, only 8.1gms, is far less than -->more ... rather than is unidiomatic
b) more as a substitute for four quaters than the dollar bill because it weighs only 8.1gms, far lighter than --> incompleted comparison structure, for should be added after than
c) as a substitute for four quarters more than for the dollar bill because it weighs only 8.1 gms, far less than --> correct, more here is an adverb modifying for used, and appropriate comparison structure: S + V + prep (as) + Object (a substitude) + prep (for) + obj (four quaters) + more than + prep (for) + obj (dollar bills) + ...

Notes: its a correct comparison structure mate, a more simple exp: He likes you more than her

d) as a substitue for four quarters more than the dollar bill because its weight of only 8.1gms is far lighter than it is for --> many errors
e) as a substitute more for four qurters rather than for the dollar bill because its weight , only 8.1gms, is far less than it is for --> mentioned error

I did not like any of the choices.
I rejected C because 1) 'more' seemed misplaced and 2) 'it' in the 'because it weighs only 8.1' can refer to both the dollar bill and the coin.

OA and source?

Answer is C.
Certainly a better choice than others.

None of the answers are good. However, C for me.

IMO A,
but does not have any confidence in any of the choices. Tough one, please post the source and oa.

Substitute of will be the right idiom to use...In A, is not clear to me about what "it" is referring to.....