In the given process, for an ideal gas ΔW=0 and ΔQ<0 By 1st law of thermodynamics, ΔQ=ΔW+ΔU where, ΔU= internal energy of gas. ⇒ΔQ=ΔU and also ΔU<0. As the internal energy depends on temperature. Therefore, if change in internal energy is negative, it mean temperature of gas is decreased.