Since BaNO3 is added in excess, all the sulfate ions have combined with Ba2+ ions to form BaSO4.
Find the number of moles of BaSO4. Now, n(BaSO4):n(SO4 2-) = 1:1 so now you know the number of moles of SO4 2-.
[SO4 2-] = n(SO4 2-)/V = n(SO4 2-)/(0.2) to get your answer.