The electric field is denoted by the potential gradient i.e. the change in potential corresponding to the displacement

If V is constant, then dV=0, that implies E is zero. Thus, option (b) is correct and option(a) is incorrect.

If there is any charge inside the region, then that charge would generate an electric field inside the region, and potential will not remain constant (dV≠0). Thus, no charge can be present inside the region. Option (c) is correct.

If a charge is placed outside the region, no change will take place in the electric field. This is because when we apply Gauss’s Law, there is no enclosed charge, thus, there will be no electric field in the region.