Is it true grapes have a powerful antioxidany and can help prevent cancer?

Yeah, but...: Grapes are good, so is grape juice, jam, wine. Foods that have color, even some that are white, have anti-oxidants. Coffee and teas have anti-oxidants. They work in the test tube. Unreasonable to expect that intake of these alone will prevent cancer. Can't undo bad habits like tobacco, inactivity, diet or alcohol abuse by taking antiox. Eat grapes, other fruits, veggies. Moderation in all things.

No proof: Grapes have anti-oxidants, however, there is no objective evidene that grapes prevent cancer.

I very much doubt it: By its nature, surveys of people's eating habits correlated with cancer prove little cause-and-effect, and the convincing animal studies just aren't there. Any mumbo-jumbo is news, even more about common food, but overall, antioxidants of any sort haven't held up as preventing cancer, and in some studies they shorten the life-span or damaged the health of those who took them.