Explanation

(b) According to first law of thermodynamics
$\mathrm{\Delta}Q = \mathrm{\Delta}U + \mathrm{\Delta}W$
For an adiabatic process, $\mathrm{\Delta}Q = 0\ \therefore\ \mathrm{\Delta}U = - \mathrm{\Delta}W$
During an adiabatic expansion, $\mathrm{\Delta}W$ is positive. Therefore $\mathrm{\Delta}U$ will be negative, so internal energy decreases and hence temperature of the system decreases
For an adiabatic process $PV^{\gamma} =$ constant
$\therefore P^{'}V^{'\gamma} = PV^{\gamma} \Rightarrow P^{'} = P\left( \frac{V}{V^{'}} \right)^{\gamma}$
As $V^{'} > V,\ \gamma > 1\ \therefore P^{'} < P$