大家觉得应该选什么阿

this one isn't that bad if you get down to the "skeleton" of the sentence. to do that, you should kill intervening prepositional phrases that don't affect the grammar of the main sentence.
to this end, you can simply get rid of the following:
"per milligram"
"for another product"
"with the same active ingredient"
and you're left with:
a new drug is being sold for three times the price ____ the drug's maker charges.
once you see this, it should be more clear that (c) is the correct answer. if you're a non-native speaker and you can't recognize "that" as the word belonging in that blank, then here's the reason: it's because "price" is the direct object of "charges". when you turn around such constructions, you use "that":
i hit the ball --> the ball that i hit
she delivered the baby --> the baby that she delivered
etc.

water wrote:
A new drug is being sold for three times the price 'of what' the drug's maker charges.

you have to read these things very, very literally.
"what the drug's maker charges" is ... guess what ... a price.
therefore, "three times the price of what the drug's maker charges" would refer to the price of a price. that is illogical.
this construction is thus not only "wordy", but actually nonsensical.

感觉太傻上说 price that =what  也挺有道理的