For interrogative sentences starting with "Who", the passive starts with "By whom". The correct structure is: By whom + auxiliary verb + object (as subject) + past participle.

১. Option (ক) is incorrect because the auxiliary "was" should come before the subject in a question.
২. Option (খ) incorrectly uses "had" and has a flawed prepositional placement.
৩. In "By whom was the work done?", the word order follows the standard interrogative passive rule.

অতিরিক্ত তথ্য: Voice change is a common topic in BCS English Language tests to evaluate grammatical accuracy.