The mean free path for a gas, with molecular diameter $d$ and number density $n$ can be expressed as:

• Correct answer
  A

  $\frac{1}{\sqrt{2}n\pi d^2}$
• B

  $\frac{1}{\sqrt{2}{n}^2\pi d^2}$
• C

  $\frac{1}{\sqrt{2}{n}^2{\pi }^2{d}^2}$
• D

  $\frac{1}{\sqrt{2}n\pi d}$

Hence, the correct answer is option: 'A'.