(A) tells me about what rational consumers do. But we’re trying to prove that a certain kind of consumer IS rational. Learning something about what rational consumers do doesn’t prove that someone is rational.

(B) tells us what’s true “whenever it is rational not to acquire detailed info about a product.” But we’re trying to prove that it’s rational not to acquire such info. Learning about what would be true WHEN it’s rational not to get such info doesn’t establish that it IS rational not to get the info.

Whether the benefits actually outweigh the costs is irrelevant, because the premise concerns someone’s EXPECTATIONS about whether the benefits outweigh the costs. What matters is what consumers who don’t get complete info EXPECT about benefits/costs.

(D) tells me about what rational consumers expect. But we’re trying to prove that a certain kind of consumer IS rational. Learning something about what rational consumers expect doesn’t prove that someone is rational.

(E) tells us that consumers who don’t get complete info do not expect the benefits of getting the info to outweigh the costs/difficulty of getting it. This triggers the rule in the premise and allows us to conclude that for these consumers, it’s rational not to acquire the info.