The sentence “this is a common belief” is a complete sentence interjected in front of a dependent clause.

While using parenthesis is not precisely wrong-it could just have easily been set off by hyphens as in this sentence-but it is still awkward. Changing the sentence to be a phrase rather than an independent (or dependent clause, for that matter) follows a rule of word economy. Why use more words when you can use fewer?

Furthermore, if you look at your choices, you are really being asked to choose which bests fits the meaning: despite/which is/which was/as is

B. Despite is not correct because it is does not directly contradict anything.

C.Which is is wordy

D.Which was is not only past tense but adds an extra (unnecessary) coordinating conjunction

E. As is commonly believed… is present tense like the rest of the sentence plus it simply fits the meaning best.

However, I would like to point out, as a former long distance swimmer, it really does help keep your body warmer.