Choice D is the best answer. This question tests dash usage for emphatic appositives.

1. Context clues: The appositive describes what the exhibition IS, with emphasis needed.
2. Meaning: Dashes set off emphatic appositives and must be used in pairs.
3. Verify: "The museum's newest exhibition—a comprehensive survey of Impressionist masterpieces—opens..." correctly brackets with emphasis.

💡 Strategy: Use paired dashes to add emphasis to appositives or parenthetical information.

Choice A is incorrect because commas work but provide less emphasis. Choice B is incorrect because punctuation is needed to set off the appositive. Choice C is incorrect because dashes must be paired.