D! 代入後:A new hair-growing drug is being sold for three times the price, per milligram, of what the drug’s maker charges for another product with the same active ingredient.
提出關鍵部分就是:the price of what............. 這不是很通順嗎?其中what實際 = the price that ,避免重復並顧及習慣用法用what 引導的是一個從句。
i think e is right, the price at which ..., at which is a prep modifier of "the drug’s maker charges for another product with the same active ingredient."
the price that..., that seems to have nothing to modify