Answer
$0.5~~$ of the maximum charge is on the capacitor.
Work Step by Step
We can write an expression for the energy stored in the electric field when the maximum charge $Q$ is on the capacitor:
$U_E = \frac{Q^2}{2C}$
We can find the charge $q$ when $25.0\%$ of the energy is stored in the electric field:
$\frac{q^2}{2C}= 0.25~\frac{Q^2}{2C}$
$q^2= 0.25~Q^2$
$q= 0.5~Q$
$0.5~~$ of the maximum charge is on the capacitor.
