Explanation
Correct answer: A.
Choice A is the best answer. "Negating" means "reversing" or "making invalid." Proving that Vermeer worked with students would reverse the view of him as a solitary artist.
Why the other choices are wrong
Choice B
Choice B is incorrect. "Prefiguring" means "being an early indicator of." There already existed views of Vermeer as a solitary painter, so a new painting would not be an early indicator of those views. Rather, a painting proving that Vermeer had a student would contradict those earlier views.
Choice C
Choice C is incorrect. "Entrenching" means "solidifying." A painting proving that Vermeer had a student would not solidify views of him as solitary, but would rather contradict those views.
Choice D
Choice D is incorrect. "Substantiating" means "supporting with proof." A painting proving that Vermeer had a student would not support views of him as solitary, but would rather contradict those views.