A series LCR circuit is connected to an ac source of 220V, 50Hz ....

The average rate at which energy supplied i.e. power delivered is maximum in case of the resonance, therefore

${X}_{C} = {X}_{L}$

$⇒ \dfrac{1}{\omegaC} = 79 . 6Ω$

$⇒ C = \dfrac{1}{\omega \times 79 . 6} = \dfrac{1}{2\pif \times 79 . 6} = \dfrac{1}{100\pi \times 79 . 6} = 40\mu F$

Hence, $C = 40\mu F$.