The Gaussian surface needed for calculating the electric field due to a charge distribution is

Explanation

Consider a sphere of radius R having total charge q uniformly distributed in its entire volume. Then, the charge density `rho=(q)/((4pi)/(3)R^(3))" "(because "volume of sphere"=(4pi)/(3)R^(3))`
We have to find the electric field at a point P at a distance r = OP from the centre O of the sphere. Due to spherical symmetry the field is radial, and hence a sphere of radius r concentric with the sphere of charge is taken as Gaussian surface. Suppose, E be the field at P.