Hey There,
First off, the dots are a way to represent that whatever goes in those boxes need to be different (because V and Z can't have anything in common). As Noah wrote at the bottom, we could conceivably do the same for the boxes above them, to represent the idea that there can't be any in common. The crossed off equal sign he wrote means the same thing, but I prefer the dots method, because it's inside the diagram.
As for what the question is asking for, this is a conditional question (it introduces a new condition), so we have to start by making new inferences. Let's look at our original diagram and see what it means if only one store has folk music.
Well, it appears that we've already got our one folk! That means none of the other stores get to have folk. First off, this means that we can fill in the rest of Store S, because it has to have three types of music. We already knew that V and Z couldn't have J, and now they can't have F either. That only leaves O and R, and we know that the two stores can't have any music in common! This means one of them must have O, and the other must have R. We don't know which one, so we'll express that in our diagram with a slash.
From here, you can see that the answer has to be B. None of the other choices are possible. (Interestingly, though the question is a "could be true" question, answer choice B MUST be true!).
Let me know if you have more questions!
-t