(a) Natural frequency is nothing but resonant frequency.
In this case $X_{L} = X_{C}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \omega_{0}L = \frac{1}{\omega_{0}C}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ {\ \ \omega}_{0}^{2} = \frac{1}{LC}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ {\ \omega}_{0} = \frac{1}{\sqrt{LC}}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ 2\pi f = \frac{1}{\sqrt{LC}}$
$\Rightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ f = \frac{1}{2\pi\sqrt{LC}}\ $