d
As we know,
$P =\frac{1}{3} \frac{ mnC ^2}{ V }$
so, $P \propto nC ^2$
when $m$ and $V$ are constants.
But $C \propto \sqrt{ T }$
As the temperature of two vessel is the same, the rms velocity of gas molecules in two vessel will be the same.
When temperature is same for two vessels then
$P \propto n$
so,
$\frac{ P _1}{ P _2}=\frac{ n _1}{ n _2}=\frac{1}{2}$