Answer:
According to Benvolio, the fight was caused by Tybalt.
Explanation:
In Benvolio's words, the fight was caused by Tybalt. She tells Lady Capulet (her aunt) that when he arrived, the Montagues servants were fighting with their servants and that he tried to separate them. However, Tybalt appears with his sword brandishing it in the air and insulting and belittling Benvolio.
They continued to fight until the Prince arrived and separated them.
Given this information we can say that According to Benvolio, the fight was caused by Tybalt.