OA is E. However since in E, "are calling" seems unparallel with the others verbs tens used. That makes me concenr.
I would also vote for E, as options C and D have other errors which cannot be ignored.
The problem in the original sentence is that its not maintaining parallelism.
"some propose to...others by suggesting..." is not parallel. Hence A and B are out for the same reason.
A. not maintaining parallelism
B. not maintaining parallelism
C. Use of verb should be preferred over use of action noun. Here "reducing" will sound better than "the reduction". Also "for decreasing" should be replaced with "to decrease".
D. Here "reducing" should be preferred over use of action noun "the reduction".
Also, this is not a parallel sentence. "some propose to...others suggest...and still others by calling". Notice the first 2 clauses using Simple Present form of verb, whereas 3rd clause "still others by calling", has no verb, instead its using a gerund.
E. Maintains parallelism with correct part of sentence "some propose to...others suggest...". Past participle form "reducing" has been correctly used instead of "the reduction".
The 3rd clause ", and still others are calling..." is using a Present continuous form. Atleast the clause has kindda similar structure.
guide says that we can have parallel clauses. Only option C and E have all the 3 as clauses, whereas, other options are introducing phrases which cannot be parallel to main clause.
But since C has some other errors, E is perfectly correct choice.