Answer
When the current is a maximum, the emf is zero.
Work Step by Step
$\mathscr{E} = \mathscr{E}_m~sin~\omega_d t$
$i_C = I_C~sin~(\omega_d t+\frac{\pi}{2})$
The emf curve and the current curve are out of phase by $90^{\circ}$
Therefore, when the current is a maximum, the emf is zero.
