Explanation

The trigger despite indicates that the reefs had not justified the environmentalist’s initial fears, so you need a word such as positive or harmless in the blank. Neither caustic nor interminable means positive or harmless, so eliminate answer choices (C) and (E). Neither unfounded, which means groundless, nor plaintive, which means mournful, makes sense in the blank; eliminate choices (A) and (F). Both benign and innocuous can mean harmless, so choices (B) and (E) give you appropriate, equivalent sentences.

Hence options B and E is correct!