Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of punctuation within a sentence. This choice correctly completes the parenthetical element "as many as 200,000, by some estimates" with a closing parenthesis, pairing with the opening parenthesis that appears earlier in the sentence. This parenthetical element functions to specify the number of residents, and the use of parentheses indicates that this element could be removed without affecting the grammatical coherence of the sentence.
Why the other choices are wrong
Choice B
Choice B is incorrect. While this choice completes the parenthetical element with a closing parenthesis, no comma is needed before the prepositional phrases ("in...size") that provide essential information about where the residents were housed.
Choice C
Choice C is incorrect because a dash can’t be paired with an opening parenthesis in this way to separate the parenthetical element from the rest of the sentence.
Choice D
Choice D is incorrect because it doesn’t close the parenthetical element that was opened earlier in the sentence.