Answer:
plays that are tragic
Explanation:
Parallelism is arranging words of identical syntactical constructions in sentences.
We need a parallel, i.e. identical, construction for the phrase plays that are funny. Obviously, the word when does not help to create parallelism in the corresponding phrase. The solution is simple: we should repeat the beginning of the phrase and change the adjective.
The result is a sentence with a parallel structure:
Plays that are funny are more popular than plays that are tragic.
