The best choice here is (Choice B) "cheese—". This selection effectively uses an em dash to introduce the explanation that follows, helping to clarify the relationship between the statement about cheese and the reason for the misconception regarding mice's affinity for it.
So the complete sentence with the best choice would be:
"Contrary to popular opinion, mice actually prefer nuts and seeds to cheese; rumors about the rodent’s affinity for dairy come from the fact that cheese—used to be stored in ways that were accessible to mice."