top.v

module top (
        input [1:0] S,
        input [7:0] A, B, C, D,
        output reg [7:0] Y
);
        always @* begin
                case (S)
                        2'b00: Y <= A;
                        2'b01: Y <= B;
                        2'b10: Y <= C;
                        2'b11: Y <= D;
                endcase
        end
endmodule