The correct answer is the following: option a. Of all the events shown in the question, the one that happens first is the fight between Tybalt and Mercutio. On Act III Scene I, Romeo meets Tybalt on the street and this last one challenges Romeo to a duel. When Romeo refuses the challenge his friend Mercutio takes his place on the fight, as he is disgusted by Romeo's cowardice. When Romeo tries to stop the fight, Tybalt kills Mercutio, and possessed by anger, Romeo kills Tybalt in revenge.