Step 1: Identify the active-voice tense. 'Had been reviewing' = past perfect continuous active. It describes an action ongoing up to a past reference point ('when the chairperson arrived'). Step 2: Form the passive. Active past perfect continuous passive uses: had + been + past participle. The auxiliary chain had been is retained; reviewing → reviewed. Step 3: Reconstruct. 'The proposals had been reviewed by the committee for three hours when the chairperson arrived.' Step 4: Verify each distractor. Was being reviewed = past continuous passive (drops had). Has been reviewed = present perfect passive (wrong tense). Were being reviewed = past continuous plural (drops had, wrong number for committee). Answer: had been reviewed.