Because an achiral molecule is symmetrical, flipping it yields a structurally identical molecule. A flipped chiral

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.

Choice A is incorrect. This choice uses a transition that indies 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 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 is incorrect. This choice uses a transition that indies 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.