Many of his novels are concerned with controversial subjects ----.
A) until the problems resulting from the war finally solved
B) that smoking is only one of them
C) as he was awarded the top prize
D) which will have surprised no one
E) which makes them extremely popular
Yet another question from a nationwide exam in Turkey. The official answer is E, but I think a comma is needed before "which" as it is a non-defining relative clause. What do you think?