Answer
$0.866~~$ of the maximum current is in the inductor.
Work Step by Step
We can write an expression for the energy stored in the magnetic field when the maximum current $I$ is in the inductor:
$U_B = \frac{L~I^2}{2}$
We can find the current $i$ when $75.0\%$ of the energy is stored in the magnetic field:
$\frac{L~i^2}{2}= 0.75~\frac{L~I^2}{2}$
$i^2= 0.75~I^2$
$i= 0.866~I$
$0.866~~$ of the maximum current is in the inductor.
