The given sentence "Who built this colossal architectural structure?" is in the active voice. We need to convert it into the passive voice. The passive voice emphasizes the action's receiver rather than the performer.

Let's analyze the options:

• Option 1 ("Who had this colossal architectural structure built?") uses a causative structure, implying someone commissioned the building, not directly constructing it. This is not a simple passive transformation.
• Option 2 ("By whom was this colossal architectural structure built?") correctly transforms the sentence into the passive voice. The focus shifts from the unknown builder ("who") to the architectural structure itself as the receiver of the action ("built"). The auxiliary verb "was" indicates a past tense passive construction.
• Option 3 ("By whom has this colossal architectural structure been built?") uses the present perfect passive, implying the building's completion is relevant to the present time. This is not the most direct passive transformation of the original sentence.
• Option 4 ("By whom is this colossal architectural structure built?") uses the present passive, which doesn't accurately reflect the original sentence's past tense.

Therefore, only option 2 accurately represents the passive voice transformation of the original question, maintaining the same meaning while shifting the grammatical focus.