Hope you are looking for this type of solution,
i) A ?B = {a, d} ? {b, c, f} = {a, b, c, d, f}
ii) A ? C = {a, d}? {a, c, e, f} = {a, c, d, e, f}
iii) B ? C = {b, c, f} ? {a, c, e, f} = {a, b, c, e, f}
iv) A ?B ? C = {a, d} ? {b, c, f } ? {a, c, e, f} = {a, b, c, d, e, f} = T