(C) is the language $ a b^{2n} $ where the $ b ^ {2n} $ can come from both $ A ~and ~ B ~ of ~ s → aAB $ Hence it is ambiguous.
(A) too has that same problem in that the string aabb can the derived from both the first or second s in $ S → SS $
(B) too is ambiguous from @SanjaySharma’s answer.
So all are ambiguous.