In answering this question, much depends on what your own beliefs are regarding the play. By Act III, a clear delineation between how individuals behave has been made. As you read through the events, which one seemed to have the most impact on either the characters or you, as the reader? This might come down to what you believe the driving force of the play is. Would the accusations and how they gain a life of their own constitute importance? Would the silence and fear which creates a sense of paranoia in Salem represent the driving force of the play? Would the courage of some and the lack of it with others represent something that sticks out in your mind? Discover what you believe would be the critical essence of the play and then find the event in Act III which highlights it. At this point, I believe you have answered the question.