'Birds' does not describe the kind of efforts that have been stepped up. 'Conservation efforts' works as a phrase in this sentence, implying that the efforts, to conserve something/someone, have been stepped up. So, (A) must be replaced with (C).
Let's check whether 'birds' fits third in the sequence of highlighted words in the sentence.
'Decline in the number of birds' implies that these conservation efforts have been stepped up because of the decline in the population of birds. This makes the sentence meaningful.
By interchanging, (A) and (C), the sentence implies that conservation efforts have been stepped up because of the declining bird population over the years because of the lack of aquatic vegetation.
Hence, option b) is correct.
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