Choice D is the best answer. This question tests dash usage for sudden qualification.

1. Context clues: The addition qualifies or questions the previous statement.
2. Meaning: A dash signals an abrupt afterthought or qualification.
3. Verify: "The negotiations had been proceeding smoothly—or so we believed" effectively adds doubt.

💡 Strategy: Use a dash to introduce an abrupt qualification or afterthought.

Choice A is incorrect because a comma doesn't convey the abruptness. Choice B is incorrect because a semicolon requires two independent clauses. Choice C is incorrect because a colon introduces explanation, not qualification.