The question requires us to determine the efficiency of an ideal heat engine working between the freezing point and boiling point of water. The Carnot efficiency is given by:
\[ \eta = 1 - \frac{T_C}{T_H} \]
where:
The boiling point of water is \( 100^\circ\text{C} = 373\,\text{K} \).
The freezing point of water is \( 0^\circ\text{C} = 273\,\text{K} \).
Substituting these values:
\[ \eta = 1 - \frac{273}{373} \]
\[ \frac{273}{373} \approx 0.7311 \]
\[ \eta = 1 - 0.7311 = 0.2689 \]
Converting to percentage:
\[ \eta \approx 26.89\% \]
Rounding to one decimal place:
\( \eta \approx 26.8\% \)