Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2017 / S19 / 3 / a / Solution

Use the explicit equation $n/n_c=0.002$; the surrounding phrase reverses the verbal ratio. It gives $n_c=2.0\times 10^{21}$ cm${}^{-3}$. From ${\omega }_0^2=n_c{e}^2/({ϵ}_0{m}_e)$,This is an underdense optical plasma. Treating the ratio as $n_c/n=0.002$ would instead contradict the specified underdense condition.