The survival of coral colonies, which are composed of innumerable tiny polyps living in a symbiotic relationship with brilliantly colored algae, are being threatened, experts say, not only by pollutants like agricultural runoff, oil slicks, and trash, but also by dropped anchors, probing divers, and global warming.
(A) are being threatened, experts say, not only by pollutants like Incorrect use of plural verb "are" for singular noun "survival". Also, incorrect use of "like" to give specific examples - "like" is used to show similarity whereas "such as" is used to give specific examples. Eliminate.
(B) are being threatened, experts say, by not only pollutants such as Incorrect use of plural verb "are" for singular noun "survival". Eliminate.
(C) is not only being threatened, experts say, by pollutants such as "not only" being applied to "threatened" means that, apart from "being threatened", something else is also happening to the survival of coral colonies, whereas the sentence is trying to state that the survival of coral colonies is being threatened by more than one thing. Change in meaning is not supported by the rest of the sentence. Eliminate.
(D) is not only being threatened, experts say, by pollutants like Same error as in (C), and incorrect use of "like" as in (A). Eliminate.
(E) is being threatened, experts say, not only by pollutants such as Correct answer - all above errors are rectified and no new errors are introduced.