I'm glad you got that D is correct.
When my students tell me they're consistently narrowing their selections down to two answer choices - the correct one and an incorrect one - and picking the incorrect one, my reaction is "That means you're almost there." Being able to pick out the correct answer choice is one hurdle - being able to see past the carefully constructed incorrect answer choices is the final hurdle.
Anyway, C is close, but not quite proven. First, we don't know for sure that they didn't select every
construction tool to be on sale...though I admit that the sentence structure seems to suggest that they did not, so this isn't the strongest evidence.
Stronger, though, is the fact that we have no information indicating that any item is not
returnable for store credit. We know sale items can only be returned for store credit and not for a refund, but we don't know that an item being returnable for a refund means that that item can't
be returned for store credit. At least, it never says so, and so it doesn't satisfy the stringent "Must Be True" test.
Hope that helps,