The correct option is B less than natural frequency Angular frequency of damped oscillation
$\omega^{\prime}=\sqrt{\omega_0^2-\left(\frac{b}{2 m}\right)^2}$
$\Rightarrow \omega^{\prime} < \omega_0$
Where natural angular frequency $\omega_0=\sqrt{\frac{ k }{ m }}$
Since frequency and angular frequency are related as,
$\omega=2 \pi f$
Hence, frequency of damped oscillation is less than natural frequency.

