The flux according to Gauss Law
where Q is charge enclosed inside the body therefore flux will not change when the radius of surface is doubled because still the charge enclosed inside the body will be same.
Electric field at the surface
therefore when the radius is doubled electric field will decrease by a factor of 4 . because now R' = 2R
==>
hence option (a) is correct i.e. The flux would not change and the electric field would decrease by a factor of four.