If the current of ' $I$ ' gives rise to a magnetic flux ' $\phi$ ' through a coil having ' $N$ ' turns, then…
If the current of ' $I$ ' gives rise to a magnetic flux ' $\phi$ ' through a coil having ' $N$ ' turns, then magnetic energy stored in the medium surrounding the coil is
$\frac{\mathrm{N} \phi \mathrm{I}}{4}$
$\frac{\mathrm{N} \phi \mathrm{I}}{2}$
$\frac{\mathrm{NI}}{2 \phi}$
$\frac{\mathrm{N} \phi}{2 \mathrm{I}}$
Solution
Inductance of the coil, $\mathrm{L}=\frac{\phi}{\mathrm{I}}$
Energy stored due to single turn,
$\mathrm{U}=\frac{1}{2} \mathrm{LI}^2=\frac{1}{2}\left(\frac{\phi}{\mathrm{I}}\right) \mathrm{I}^2=\frac{\phi \mathrm{I}}{2}$
Energy stored due to $\mathrm{N}$ turns,
$\mathrm{U}^{\prime}=\frac{\mathrm{N} \phi \mathrm{I}}{2}$