Answer
At $~~t = 1.25\times 10^{-6}~s~~$ the inductor will have a maximum magnetic field.
Work Step by Step
We can find the period:
$T = \frac{1}{f} = \frac{1}{2.0\times 10^5~Hz} = 5.0\times 10^{-6}~s$
At $~~t = \frac{T}{4}~~$ the magnetic field in the inductor will reach a maximum.
At $~~t = 1.25\times 10^{-6}~s~~$ the inductor will have a maximum magnetic field.
