Choice D is the best answer. This question tests dash usage for abrupt shift in thought.

1. Context clues: There's a dramatic shift from success to sudden downturn.
2. Meaning: A dash signals an abrupt change or dramatic turn.
3. Verify: "The company's profits had reached record levels—then came the unexpected market crash" conveys the sudden shift.

💡 Strategy: Use a dash to signal an abrupt change in thought or unexpected turn.

Choice A is incorrect because a comma doesn't convey the dramatic shift. Choice B is incorrect because a semicolon suggests a more measured connection. Choice C is incorrect because a colon typically introduces explanation, not abrupt change.