The correct option is C Were bags being carried by them?
When the active voice is changed to passive voice, the object and the subject of the sentence interchange their positions, the focus is on the action of the verb received by the subject. The verb of the sentence changes to passive form: auxiliary verb + past participle. However, the tense of the sentence is not changed.
Option C is correct as it correctly converts the active to passive while keeping the past continuous tense of the sentence intact.
Options B and D are incorrect as they are not in the past continuous tense. Option A is incorrect as well because it has been changed into an assertive sentence when the sentence in the question is interrogative.