Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is pronoun-antecedent agreement. The singular pronoun "it" agrees in number with the singular antecedent "water" and clearly indicates that the water swirls into a whirlpool.
Why the other choices are wrong
Choice A
Choice A is incorrect. The plural pronoun "these" neither agrees in number with the singular antecedent "water" nor clearly indicates that the water—not another plural noun in the sentence, such as "landmasses" or "winds"—swirls into a whirlpool.
Choice B
Choice B is incorrect. The plural pronoun "those" neither agrees in number with the singular antecedent "water" nor clearly indicates that the water—not another plural noun in the sentence, such as "landmasses" or "winds"—swirls into a whirlpool.
Choice C
Choice C is incorrect. The plural pronoun "them" neither agrees in number with the singular antecedent "water" nor clearly indicates that the water—not another plural noun in the sentence, such as "landmasses" or "winds"—swirls into a whirlpool.