Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Efficiency of a reversible heat engine will be highest at $$-273°$$C temperature of cold reservoir.
Reason R: The efficiency of Carnot's engine depends not only on temperature of cold reservoir but it depends on the temperature of hot reservoir too and is given as $$\eta = \left(1 - \frac{T_2}{T_1}\right)$$
In the light of the above statements, choose the correct answer from the options given below:

We are given an Assertion-Reason question about the efficiency of a reversible (Carnot) heat engine.

Assertion A: Efficiency of a reversible heat engine will be highest at $$-273°C$$ temperature of cold reservoir.

Reason R: The efficiency of a Carnot engine depends on both the hot and cold reservoir temperatures, given by $$\eta = 1 - \dfrac{T_2}{T_1}$$.

The efficiency of a Carnot engine is $$\eta = 1 - \dfrac{T_2}{T_1}$$, where $$T_2$$ and $$T_1$$ are the cold and hot reservoir temperatures in Kelvin. When the cold reservoir is at $$-273°C = 0$$ K, we get $$\eta = 1 - \dfrac{0}{T_1} = 1 = 100\%$$, which is the maximum possible efficiency. So Assertion A is true.

The Reason states the correct formula and correctly notes that efficiency depends on both reservoir temperatures. So Reason R is also true.

Now, since the formula in R directly explains why the efficiency is maximum when $$T_2 = 0$$ K (substituting gives $$\eta = 1$$), R is the correct explanation of A.

Hence, the correct answer is Option D.