The mean free path of gas molecules depends on $(d =$ molecular diameter$)$

The vapour of a substance behaves as a gas:

If the pressure in a closed vessel is reduced by drawing out some gas,the mean free path of the molecules

$A$ gas deviates maximum from the ideal gas law at

At a temperature of $314 \,K$ and a pressure of $100 \,kPa$, the speed of sound in a gas is $1380 \,ms^{-1}$. The radius of each gas molecule is $0.5 \,Å$. The frequency of sound at which the wavelength of the sound wave in the gas becomes equal to the mean free path of the gas molecules is (Boltzmann constant $k = 1.38 \times 10^{-23} \,JK^{-1}$):

What is mean free path?