Choice D is the best answer. This question tests dash usage for a sudden break or change in direction.

1. Context clues: There's an abrupt shift from "seemed perfect" to discovering problems.
2. Meaning: A dash signals an abrupt turn or unexpected qualification.
3. Verify: "The proposal seemed perfect on paper—until we examined..." conveys the sudden change.

💡 Strategy: Use a dash when there's a sudden shift or dramatic turn in the sentence.

Choice A is incorrect because a comma doesn't capture the abrupt shift. Choice B is incorrect because a semicolon requires two independent clauses. Choice C is incorrect because a colon introduces elaboration, not unexpected turns.