Where can we say that Benvolio is a foil to Mercutio in Romeo and Juliet?

Let us remind ourselves that a foil is a character that is
used to highlight the characteristics of another character by comparison. Therefore, the
humour of a character can be exaggerated by placing him next to an extremely boring or
unhumorous character. You are right in identifying that Benvolio at certain parts of the
play acts as a foil to Mercutio, and this scenario of wit is certainly one of the traits
of Mercutio that is emphasised through this technique. One scene where we can see this
in operation in Act II scene 4, when Mercutio and Romeo engage in a series of puns and a
battle of wits. In the verbal duel that follows, interestingly Benvolio does not at all
participate, except to make a final comment. His silence and his obvious lack of ability
to engage in a verbal duel only makes Mercutio's quick tongue and wit all the more
obvious.