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=(1-T_(2))/(T_(1))`
In the light of the above statements,s choose the correct answer from the options given below :

Options

A. A is true but R is false
B. Both A and R are true and R is the correct explanation
C. Both A and R are true but R is NOT the correct explanation of A
D. A is false but R is true

Answer

To solve the given question, we need to analyze the Assertion (A) and Reason (R) provided in the context of the efficiency of a reversible heat engine, specifically a Carnot engine.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the efficiency of a reversible heat engine will be highest at -273°C, which is equivalent to 0 K (absolute zero).
- According to the Carnot theorem, the efficiency (η) of a Carnot engine is given by the formula:
\[ \eta = 1 - \frac{T_{\text{cold}}}{T_{\text{hot}}} \]
- If the cold reservoir temperature \(T_{\text{cold}}\) is at absolute zero (0 K), the formula simplifies to:
\[ \eta = 1 - \frac{0}{T_{\text{hot}}} = 1 \]
- This indicates that the efficiency would be maximum (100%) if the cold reservoir is at absolute zero.

2. **Understanding the Reason (R)**:
- The reason states that the efficiency of a Carnot engine depends on both the temperature of the cold reservoir and the temperature of the hot reservoir.
- The formula \(\eta = 1 - \frac{T_{\text{cold}}}{T_{\text{hot}}}\) indeed shows that the efficiency is influenced by both temperatures.
- Thus, the reason is correct as it explains that the efficiency is not solely dependent on the cold reservoir temperature but also on the hot reservoir temperature.

3. **Conclusion**:
- Both the assertion and the reason are true.
- Furthermore, the reason provides a correct explanation for the assertion since the efficiency reaching its maximum at absolute zero is due to the relationship defined in the Carnot efficiency formula.

### Final Answer:
- Both A and R are true, and R is the correct explanation of A.