Given here: $L=0.5\times {10}^{-3} \mathrm{H}$

$C=200\times {10}^{-6} \mathrm{F}$

$R=2 \mathrm{\Omega }$.

If current is in phase with emf, so this is the condition of resonance.

$f=f_R=\frac{1}{2\pi \sqrt{LC}}=\frac{1}{2\times 3.14\sqrt{LC}}$

$f=\frac{1}{6.28\sqrt{0.5\times {10}^{-3}\times 200\times {10}^{-6}}}=503.55 \mathrm{Hz}=5.03\times {10}^2 \mathrm{Hz}$