The correct passive construction for a ‘Who’ question in the past tense is “By whom + auxiliary verb (were) + object (you) + past participle (taught) + remaining part (French)?”.

১. In interrogative passive sentences, the auxiliary verb must come before the subject (you).
২. Option A is incorrect because the word order is that of a statement, not a question.
৩. Option C is grammatically incorrect for an formal interrogative transformation.

অতিরিক্ত তথ্য: ‘Who’ changes to ‘By whom’ in passive voice transformations.