Choice D is the best answer. This question tests dash usage for clarifying elaboration.

1. Context clues: What follows specifies what the "unexpected results" were.
2. Meaning: A dash can introduce dramatic clarification, especially for surprising information.
3. Verify: "The experiment yielded unexpected results—a complete reversal..." creates effective emphasis.

💡 Strategy: Use a dash to introduce emphatic or surprising clarification of what precedes it.

Choice A is incorrect because a comma doesn't provide sufficient emphasis. Choice B is incorrect because a semicolon requires two independent clauses. Choice C is incorrect because while a colon works, a dash better conveys the surprise.