CC,
The museum displays only certain sorts of works. Thus, if it's a work displayed, it must be a twentieth-century work and either on loan from private collectors or part of the permanent collection. Thus:
displayed
20thC
displayed
loan or permanent collection
Anything in the permanent collection has prints sold in the store:
permanent collection
print sold
Further, some things not in the collection also have prints sold
print sold
permanent collection
And Hopper's Nighthawks is one of those.
The question is Must Be True, so we need something we can infer from the facts in the stimulus.
Answer choice (A) is not correct because we know some prints are not in the collection, but we don't know that they're all prints of things on display in the museum.
Answer choice (B) is not correct because, again, some prints are not in the collection, so they don't necessarily need all the restrictions of works displayed.
Answer choice (C) is the correct answer. If something is displayed but not on loan from a private collector, it has to be in the permanent collection (the second conditional I wrote says this). The third conditional says that all such things have prints sold.
Answer choice (D) contains new information.
Answer choice (E) also contains new information.
Robert Carroll