Deriving SOP form is a good approach, but it didn’t clicked for me. I tried this approach. It’s basically elimination method.
Give inputs as S=0 and y=0 to all options, for these inputs S+ should be 1.
(A) S+ = 1
(B) S+ = 0 , eliminated.
(C) S+ = x.(1) = x, here the output depends on x but as (S = 0) state don’t have production for input ‘x’ , that’s why it’s eliminated too.
(D) S+ = 0 , eliminated.
So, option (A) is correct.