b
${E_{total}} = \frac{f}{2}NkT$ $\Rightarrow$ $T \propto \frac{1}{N}$ [$\because$ ${E_{total}} , f, k =$ determinant]
$\Rightarrow$ $\frac{{{T_2}}}{{{T_1}}} = \frac{{{N_1}}}{{{N_2}}} = \frac{1}{2}$ $\Rightarrow$ ${T_2} = \frac{{{T_1}}}{2}$
and $PV = NkT$ $\Rightarrow$ $P \propto NT$
$\Rightarrow$ $\frac{{{P_1}}}{{{P_2}}} = \frac{{{N_1}}}{{{N_2}}}.\frac{{{T_1}}}{{{T_2}}} = \frac{1}{2} \times \frac{2}{1} = \frac{1}{1}$