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 <u>when</u> 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:
<u>Plays that are funny</u> are more popular than <u>plays that are tragic</u>.
I think you mean a multiple choice question the answer should be: <span>A. The huge white van full of holiday packages for everyone came down the street.</span>