NEET 2020 Physics question on Kinetic Theory of Gases

1. A.
   $\dfrac{1}{\sqrt{2}\,n\pi d^2}$
2. B.
   $\dfrac{1}{\sqrt{2}\,n^2\pi d^2}$
3. C.
   $\dfrac{1}{\sqrt{2}\,n^2\pi^2 d^2}$
4. D.
   $\dfrac{1}{\sqrt{2}\,nd}$

[NEET 2020]