The correct answer is option 3 - inside the sphere.

When a hollow metal ball carries an electric charge, the charge is distributed across the surface of the sphere. This is known as electrostatic shielding.

Inside the hollow metal sphere, the electric field is zero. This is because the charges on the inner surface of the sphere rearrange themselves in such a way that they cancel out the electric field of the charges on the outer surface. This cancellation occurs at every point inside the hollow sphere.

On the surfaces of the sphere, there is an electric field present. However, since the question asks specifically about points, not the entire surface, option 2 - on its surface, is not correct.

Option 1 - outside the sphere, is also incorrect because the presence of the charges on the surface of the sphere will create an electric field in the surrounding space.

Option 4 - only at the center - is also incorrect because the electric field is zero at every point inside the hollow sphere, not just at the center.

Therefore, the correct answer is option 3 - inside the sphere.