a
Temperature of $O _{2}=273+15=298 K$
Temperature of $N _{2}= x$
As $P \propto n T$, as pressure is same.
$\Rightarrow n _{1} T _{1}= n _{2} T _{2} \Longrightarrow\left(\frac{1}{32}\right)(298)=\left(\frac{1}{28}\right)( x )$
$\Longrightarrow x =\frac{28}{32} \times 298=260.75 K$
Temperature in ${ }^{\circ} C =260.75-273=-13^{\circ} C$