Explanation
Correct answer: B.
Choice B is the best answer. This sentence compares a chiral molecule to an achiral one. It discusses how when a chiral molecule is flipped, it results in something very different than when an achiral molecule is flipped. So the transition "by contrast" fits the context perfectly.
Why the other choices are wrong
Choice A
Choice A is incorrect. This choice uses a transition that indicates a restatement of the same idea in different words. But this sentence doesn’t restate the same idea as the previous sentence. Instead, it makes a new point about a different type of molecule (chiral instead of achiral).
Choice C
Choice C is incorrect. This choice uses a transition that introduces an example, which doesn’t make sense here. The second sentence isn’t an example of the first sentence’s claim about achiral molecules: it actually introduces an entirely different idea that focuses on chiral molecules.
Choice D
Choice D is incorrect. This choice uses a transition that indicates the addition of an agreeing idea. But this sentence shows a contrast with the first sentence—namely, that a chiral molecule acts very differently from an achiral molecule when flipped.