I'm going to go against Msherman in his explanation above of why B does not resolve the paradox. I think understanding B is definitely the key to this problem.
So, 4 of the 5 answers should give you a POSSIBLE reason why the bookstores could be experiencing increased profits in light of the decrease in books read. D and E people don't seem to be having a problem with understand, so I'll ignore those. First I'll explain C as two people asked above:
C: Reconciles the increased profits with less reading, by showing an alternative income stream for bookstore owners. In many ways, it's not much different from E, E shows that they could POSSIBLY be (remember, this is the key in resolving the paradox, you're showing a POSSIBLE resolution to the paradox, not an absolute resolution) receiving more income from increased magazine sales. In the same way, they could POSSIBLY be receiving more money by opening coffee bars.
Now, I think eliminating B is much more simple than msherman posted above. Here was my reasoning:
B: Answer B is showing not that antitheft equipment is a possible reason for increased profits, remember we're looking for a possible reason why profits were increased, BUT one possible reason why profits DID NOT DECREASE.
I hope that helps