d
(d)Magnetic potential at a distance d from the bar magnet on it's axial line is given by \(V = \frac{{{\mu _0}}}{{4\pi }}.\frac{M}{{{d^2}}}\) \(⇒\) \(V \propto M\) \(⇒\) \(\frac{{{V_1}}}{{{V_2}}} = \frac{{{M_1}}}{{{M_2}}}\) \(⇒\) \(\frac{V}{{{V_2}}} = \frac{M}{{M/4}}\) \(⇒\) \({V_2} = \frac{V}{4}\)