a. he knows that if he says Mercutio started the fight, then the prince will be less sympathetic to him.
Explanation:
This is the most accurate answer. At the beginning of the scene, when Tybalt walks in, it is Mercutio who taunts him. However, as the fighting begins, both Mercutio and Tybalt end up dead. When Benvolio retells the story to the Prince, after Romeo has escaped, he avoids mentioning that Mercutio was the one who started the fight. Instead, he focuses on how Romeo wanted to maintain the peace. It is likely that Benvolio does so because otherwise the Prince would be less sympathetic to him.
