The conclusion of the argument made in the passage is that "rapid population growth can be disastrous for a small city." We are looking for an answer choice that will strengthen this argument.
Answer choice (C) says "some large cities can absorb rapid population growth more readily than many small cities can."
This is incorrect because it is irrelevant and out of scope. We are not concerned with large cities. The fact that some large cities would do better than most small cities does not add anything to the claim that rapid population growth can be disastrous for a small city. Perhaps large cities do excellently and small cities just do well. This would be consistent with (C) but would not strengthen the claim made in the passage.
Does that make sense? Hope it helps! Please let us know if you have any other questions!