b
(b)
Initial energy of gas = Final energy
Let K.E. of each molecule initially be $E_0$
$\therefore$ Total kinetic energy $=E_0 \times n$
Let final kinetic energy of each molecule be $E_f$
$E_0 \times n=E_f \times 2 n$
$E_f=\frac{E_0}{2}$
Since temperature is the average kinetic energy of molecules.
$T_0=K E_0 \quad T_f=\frac{K E_0}{2}$
$\therefore$ Temperature becomes $T_f=\frac{T_0}{2}$