Question

To increase the resonant frequency in series $LCR$ circuit,

Answer 1

Resonant frequency $=\frac{1}{\sqrt{LC}}={\omega }_0$

$\Rightarrow$ If we decrease $C,{\omega }_0$ would increase.

$\Rightarrow$ Another capacitor should be added in series.

Hence, $\left(C\right)$ is the correct answer.