Answer
The magnetic flux through the inductor reaches a maximum at $\frac{T}{4}$
Work Step by Step
The magnetic flux is proportional to the current through the inductor.
At $t=0$, the capacitor is charged, so $U_E$ is a maximum and the current $i = 0$
According to the diagram on page 904, the current $i$ then starts to increase until it reaches a maximum at $\frac{T}{4}$
The magnetic flux through the inductor reaches a maximum at $\frac{T}{4}$
