An action in a sentence can be represented in two ways, namely active and passive voice.
The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in active voice.
The given sentence is in the active voice.
The correct answer is C) What was bought by you?
Option C) is correct as the subject of the given sentence(you) becomes the object and vice versa. Also, the main verb (buy) is used in the third form (bought).
Options A, B, and D are not correct as the given sentence is in the past tense, not in present continuous and past continuous tense respectively.