ThisIsWater
Why "which" in this sentence can refer to animals, the only logical antecedent is "Colonies" as each animal cannot have both female and male animals in it, Is my reasoning correct?
Well, yeah. But notice that it required an unpleasant amount of work to arrive at that conclusion. If the sentence had said, "naked mole rats form colonies of approximately 20 animals, each of which has sharp teeth and bad breath," you probably wouldn't have hesitated to link "each of which" to the nearest noun, "animals." That's a natural first instinct - at least it was for me.
So to make sense of the phrase "naked mole rats form colonies of approximately 20 animals, each of which consists of a single reproductive female and workers that defend her," you have to overrule your first instinct, conclude that "animals" don't each consist of a female and workers, and then go searching for something earlier in the sentence that works better. And that's pretty confusing. Compare that with the OA: "Naked mole rats form colonies of approximately 20 animals, each
colony consisting of a single reproductive female and workers that defend her." No work required: it's 100% clear, because the sentence tells us EXACTLY what consists of a "female and workers...".
Put another way: (A) isn't necessarily WRONG, exactly, but (C) is undoubtedly
better. On the GMAT, clear and logical will always trump murky and ambiguous.
I hope that helps!