Answer
$C$ should be decreased.
Work Step by Step
The emf curve reaches a maximum value slightly before the current curve reaches a maximum.
In this case, $X_L \gt X_C$
Note that $X_C = \frac{1}{\omega_d~C}$
To increase the rate at which energy is transferred to the resistive load, we should decrease $X_C$ to get closer to resonance. Note that resonance occurs when $X_L = X_C$
Therefore, $C$ should be decreased.
