c
$\eta=25 \% \quad \eta=\frac{\mathrm{T}_{1}-\mathrm{T}_{2}}{\mathrm{T}_{1}} \times 100 \%$
$\therefore \frac{80}{\mathrm{T}_{1}} \times 100=25 \quad \Rightarrow \mathrm{T}_{1}=320 \mathrm{K}$
$\therefore \mathrm{T}_{2}=320-80=240 \mathrm{K}$
$=-33^{\circ} \mathrm{C}$