Question #148243

The correct answer is "as" because the sentence is making a comparison of equality in a negative form. The structure for such comparisons is "not so... as" or "not as... as." In this case, "so populated" is being compared to a past state (ten years ago), so the sentence should read: "Karachi is not so populated as it was ten years ago." The word "than" is used for comparisons of inequality (e.g., "more than"), "like" is used for similarity but not in this structure, and "since" indicates time, which doesn't fit here. Therefore, "as" is the only grammatically and contextually correct option.