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So the answer is indeed D here.

A is incorrect because filterfeeding in bivalves is only discussed in the second (relatively short) paragraph, which makes this unsuited as a title of a huge, detailed passage like this one.

B is also incorrect because plankton is only mentioned once, in the very end of paragraph 1: ..and the floating larval stages form a very important part of the zooplankton that sustains life in the waters...

C is wrong because there's really not much talk about prey and predator or the tropical seas themselves. The molluscs don't really "prey" on anything, they sit in a place and filter water! Plus, "tropical seas" are only mentioned in the very end of the third paragraph.

D is correct because it encapsulates much of what's said in paragraph 1 (molluscs are both constructive and destructive forces in the reef), paragraph 2 (molluscs filter water and minute organisms) and 3 (molluscs farm certain small creatures).

E is wrong because there's no mentioning of Coelenterates. If you can believe it, I learned about Coelenterates in 6th grade - my biology teacher was kindda crazy and had us learn scientific terms for animals and plants! Euglena Viridis, Paramoecium Caudatum... Still remember these!

C

A - Feeding is not the only part of symbiosis as well as the latter is not about only feeding.
D - there are at least two roles described: Prey and Predator;
B - would be good title for the third paragraph
E - too general and broad