A.: Was there any trouble at the yesterday’s demonstration? B.: Yes, about twenty people ___ . ?

In the dialogue presented, the context is the aftermath of a demonstration that took place "yesterday." The question implies that the speaker wants to know about events that occurred during that specific event, which gives us clues about the appropriate tense to use.

Option D, "were arrested," is the past simple passive voice, which is suitable here because it describes an action that was completed at a specific time in the past (yesterday). It indicates that twenty people were subjected to the action of arresting, with the focus on the occurrence of this event itself.

Option A, "had been arrested," uses the past perfect tense, which implies a prior action that occurred before another action in the past. Since the original question concerns a single event that is past, this option might imply an unnecessary complication regarding the timing.

Option B, "arrested," is grammatically incomplete as it lacks a helping verb and fails to express the action's past context.

Option C, "were being arrested," is in the past continuous passive voice and implies ongoing action, which doesn’t accurately capture the completed event that occurred during the demonstration.

Thus, "were arrested" is the most appropriate choice, as it captures the essence of a completed action that aligns with the context and timing of the discussion.