A series RLC circuit resonate at 200 Hz. If the capacitance is increased to four times, the circuit will be resonant at

In a series RLC circuit, the resonant frequency is given by

$(f = \frac{1}{{2\pi \sqrt {LC} }})$

If the capacitance is increased by 4 times, the resonant frequency will become half of the previous value.

Hence the resonant frequency = 200/2 = 100 Hz