Fill in the blank with the most appropriate word. A : Was there any trouble at the yesterday’s demonstration? B : Yes, about twenty people _______.

The correct answer is: D. were arrested

The sentence is in the past tense, so the verb must also be in the past tense. The other options are not in the past tense.

• Had being arrested is not a grammatically correct sentence.
• Arrested is the simple past tense of the verb “to arrest.”
• Were being arrested is the present progressive tense of the verb “to arrest.” It is used to describe an action that is happening at the time of speaking or writing. In this case, the action is not happening at the time of speaking or writing, so the present progressive tense is not appropriate.

Therefore, the correct answer is D. were arrested.