(a) For resonance amplitude must be maximum which is possible only when the denominator of expansion is zero
$i.e.\ a\omega^{2} - b\omega + c = 0\ \Rightarrow \omega = \frac{+ b \pm \sqrt{b^{2} - 4ac}}{2a}$
For a single resonant frequency, $b^{2} = 4ac$