Explanation
Correct answer: B.
Choice B is the best answer. The sentence acknowledges a complication affecting travel through wormholes, noting that the matter traveling through a wormhole would need to have less energy than empty space and that such matter has not been shown to exist.
Why the other choices are wrong
Choice A
Choice A is incorrect. The sentence notes that Einstein’s theory doesn’t explain how matter with negative energy density can travel through wormholes; it doesn’t acknowledge a complication affecting travel through wormholes.
Choice C
Choice C is incorrect. While the sentence acknowledges a complication affecting travel through wormholes, it misrepresents information from the notes; matter traveling through wormholes, not the wormholes themselves, would need to have negative energy density.
Choice D
Choice D is incorrect. The sentence presents a misleading interpretation of Einstein’s theory (confusing the definition of a wormhole with a condition a wormhole must fulfill); it doesn’t acknowledge a complication affecting travel through wormholes.