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

1. Context clues: There's a sudden, dramatic shift from near-completion to failure.
2. Meaning: A dash signals an abrupt change or unexpected turn.
3. Verify: "The project was nearly complete—then everything fell apart" conveys the sudden shift.

💡 Strategy: Use a dash before an abrupt qualifier or unexpected turn.

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