(a) and (b) are unidiomatic
(e) is bad diction
(d), absurd meaning: it says "the price of what the drug's maker charges".
but you must notice that "what the drug's maker charges" is already the price of the product. therefore, "the price of what the drug's maker charges" would illogically refer to the price of a price.
that leaves (c),
GGG (Gym / GMAT / Girl) -- Be Serious
Its your duty to post OA afterwards; some one must be waiting for that...