a
(a)
$1$ mole of an ideal gas has $\sim 10^{24}$ molecules and it occupies $22 L$ or $\sim 22 \times 10^{-3} m ^3$ of space.
So, average spacingbetween molecules is
$\text { nearly } d =\left(\frac{V}{N}\right)^{\frac{1}{3}}=\left(\frac{22 \times 10^{-3}}{10^{24}}\right)^{\frac{1}{3}}$
$\approx 3 \times 10^{-9} \,m$
$\text { or } d=3 \,nm$
So, order will be $1 \,nm$.