the easiest way to identify passively voice is by checking out your verbs. choice A has "fought," choice B has "brought," choice C has "was filled," and choice D has "inspired." choices A, B, and D are all in active voice--the verb moves the action forward, and the subjects perform the listed verbs. the firefighters fight, the acrobats bring joy, the teacher inspires. choice C, however, has "was filled." rather than saying "the concerned citizens filled the town hall" (active voice, where the verb directly applies to the citizens and states the action the citizens are performing), it gives you a roundabout sentence. "the town hall was filled" instead gives the verb, which SHOULD apply to the citzens, to the town hall. C is the choice which uses passive voice.
