Coral reefs are one of the most _____________ , biologically complex, and diverse marine ecosystems on Earth. This ecosystem is one of the _____________ paradoxes of the biosphere: how do clear, and thus nutrient-poor, waters support such _____________ and productive communities?

The first blank is an adjective that describes coral reefs, all of 'common', 'fragile' and 'frail' are suitable words in this context. 'deep' is better suited with the ocean or the sea, and not coral reefs. The second blank has a word which is an adjective describing 'paradoxes', the words 'sensitive' and 'unrecognised' are not suitable given the context, both 'fascinating' and 'strange' are suitable. This has already left us with only one good option, which has both correct uses for blank 1 and blank 2, but blank 3 can still be examined. In blank 3, only the words 'fruitful' and 'prolific' are suited which can both serve as adjectives to a plant which produces fruits or organisms that are productive. The most suitable option is option B.