Explanation

Creating a filter: Supposing that we're not sure what to make of the prompt, or can't create a filter, we can head straight to the answer choices.

Finding objective defects: The answer choices fall into two groups, one starting with "whether" and the other starting with "of." Here, "whether" is correct, as it is the proper way to introduce a phrase that is a yes or no question. "Of" would indicate that we are questioning the animal population, which is not under cross-examination here.

That leaves us with (A) and (B). One must be objectively in error. The "that" in (A) will be either required or incorrect. What is that relative pronoun standing for? Population, apparently. That would mean, "The annual population of the species doubled the population of the 20,000 estimate of the previous year." You can't have a population of 20,000 estimate, so (A) is incorrect. The correct answer is (B).