non-defining relative clause

[diamondrg]

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?

[Casiopea]

Hello diamondrg. Try re-wording it.

The reason his novels are extremely popular is because the subjects are controversial.

[diamondrg]

Try re-wording it.

The sentence does not belong to me. It is from a nationwide exam.

[Casiopea]

Sorry. More clearly, if you re-word the sentence, you'll be able to see why E) is the best choice.

[rewboss]

diamondrg, I think you're right: a comma is necessary.