The breadth and depth of his masterwork stunned most of Salk’s contemporaries and perplexed many of them; indeed, in private, some admitted they found the work to be__________.

From the words given below, only recondite and abstruse are similar in meaning. Although confounding could fit but we do not have a word similar to it.

Thus, recondite and abstruse are the correct answer.

IMO E & F

Hope this helps!