Answer
See the detailed answer below.
Work Step by Step
We know that the power supplied by the emf is given by
$$P_{\rm source}= I_{\rm rms}\varepsilon_{\rm rms}\cos\phi $$
where $\phi=\tan^{-1}\left[\dfrac{X_L-X_C}{R}\right]$.
So to increase the power delivered to the circuit by the emf, we have to increase the rms current which we can do in 2 ways:
1- Decreasing the resistance $R$.
2- Making $X_L=X_C$ which make $\phi=0$ which maximizes $\cos\phi=1$
