The efficiency of a Carnot's engine, working between steam point and ice point, will be

We need to find the efficiency of a Carnot engine operating between the steam point and the ice point. The steam point corresponds to the hot reservoir at $$T_H = 100°C = 373$$ K, and the ice point corresponds to the cold reservoir at $$T_C = 0°C = 273$$ K.

The efficiency of a Carnot engine is given by $$\eta = 1 - \frac{T_C}{T_H}$$. Substituting the temperatures yields $$\eta = 1 - \frac{273}{373} = \frac{373 - 273}{373} = \frac{100}{373}$$.

Evaluating this result gives $$\eta = \frac{100}{373} = 0.2681 = 26.81\%$$. Therefore, the correct answer is Option A.