It is not true that if you eat chocolate or chew gum while studying and then do the same during the test, you will remember more. This is just what some people think or believe in, but the answer is no.
Well, they do keep you more focused and apply to your concentration. Although, I do not believe it helps you remember information during the test. We got to chew gum and have snacks while taking our standardized tests, and my teacher said it was for concentration. I think that chewing gum especially helps with this. Hope I helped, and good luck :)
I have no idea, but if I had to guess out of the two, I'd pick chocolate. Can't see how chewing gum would help. On Dr. Oz the other day, he said the best thing for your brain is potassium. So maybe that's the way to go. (Foods with potassium; capsules or tablets.)