Explanation

(a) Efficiency $\eta = \frac{1 - T_{2}}{T_{1}}$
where, $T_{2} =$sink temperature,
${\ \ \ \ \ \ \ \ \ \ \ \ T}_{1} =$source temperature.
For 100% efficiency, $\eta = 1$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{T_{2}}{T_{1}} = 0$
$\Rightarrow \ \ \ Either\ T_{1} = 0\ K\ T_{2} = 0\ K$