d
$PV = nRT$
$\frac{ PV }{ nT }= R \quad$ (R here is a constant)
$ \frac{ P _{1} V _{1}}{ n _{1} T _{1}}=\frac{ P _{2} V _{2}}{ n _{2} T _{2}}$
$P _{1}=760 mm$ of $Hg$
$V _{1}=22400 cm ^{3}$
$T _{1}=273 K$
$P _{2}=10^{-11} mm$ of $Hg$
$T _{2}=300 K$
$n _{1}=6.02 \times 10^{23}$
$\frac{ n _{2}}{ V _{2}}=\frac{ P _{2} n _{1} T _{1}}{ P _{1} V _{1} T _{2}}$
$\frac{ n _{2}}{ V _{2}}=\frac{10^{-11} \times 6.02 \times 10^{23} \times 273}{760 \times 22400 \times 300}=3.32 \times 10^{5} \;molecules / cm ^{3}$