The passage clearly talks about the role Mangroves play in some of the coastal food chains. Other food chains - with or without Mangroves - might exist but they are not mentioned in the passage. So (c) is correct. The passage does not say that no food chain is possible without Mangroves so (a) is wrong. (b) is also wrong as the passage does not talk about Mangroves being an essential component of all marine ecosystems. Further (d) is also wrong for the same reasoning as the passage does not talk about the composition of marine flora and fauna.