bubbliiiiiii, usually the GMAT does not expect you to be a comma-expert, however, in this sentence one little comma really does make all the difference.
WITH this comma, the sentence containing much CANNOT be correct. This is because "voters" is a countable plural noun and thus requires "many". The comma in this case would break the connection between "however" and "much". These two words actually belong together! "However much" is very close in meaning to "despite".
WITHOUT the comma, as in the correct version of the question, there are two possibilities for the sentence to end.
"However much US voters may agree, there is a waste..." -> means despite their strong agreement something happens.
"However many US voters may agree, there is a waste..." -> means despite the number of US voters who agree, something happens.
Both of these sentence are grammatically correct. BUT the second one does not even show up in the answer choices. All answer choices except for E suggest that it is the degree of agreement that is under discussion here, and NOT the number of people who agree.