which of the following does not represent the brook? Pearl, cleansing, escape or division between illusion and reality?

Pearl certainly represents a lot in the book. Cleansing is a motif as well. That leaves "escape or division between illusion and reality".

but which of those is not represented by the brook?

You could connect all of these in the book but If I had to pick, I'd go with "escape or division between illusion and reality" as the one least representative in the book.

not the book, the brook! where pearl goes when hester and dimmesdale are talking etc.

Oh, the brook! Sorry, it's morning here. Okay that would be "Pearl".