Assertion: If `H_(2)` and `Cl` enclosed separately in the same vessel exert pressure of `100` and `200 mm` respectively, their mixture in the same vessel at the same temperature will exert a pressure of `300 mm`
Reason: Dalton's law of partial pressures states that total pressure is the sum of partial pressures.

Options

A. If both (`A`) and (`R`) are correct and (`R`) is the correct explanation of (`A`).
B. If both (`A`) and (`R`) are correct, but (`R`) is not the correct explanation of (`A`).
C. If (`A`) is correct, but (`R`) is incorrect.
D. If (`A`) is incorrect, but (`R`) is correct.

Explanation

To solve the question, we need to analyze the assertion and the reason given, and then determine their validity based on Dalton's law of partial pressures.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that if hydrogen (H₂) and chlorine (Cl₂) are enclosed separately in the same vessel, exerting pressures of 100 mm and 200 mm respectively, their mixture will exert a pressure of 300 mm at the same temperature.
- This means we need to consider the individual pressures of the gases when they are mixed.

2. **Understanding the Reason**:
- The reason provided is based on Dalton's law of partial pressures, which states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of each gas in the mixture.
- According to this law, if we have two gases, the total pressure (P_total) can be calculated as:
\[ P_{\text{total}} = P_{H_2} + P_{Cl_2} \]

3. **Applying Dalton's Law**:
- From the assertion, we have:
- \( P_{H_2} = 100 \, \text{mm} \)
- \( P_{Cl_2} = 200 \, \text{mm} \)
- Applying Dalton's law:
\[ P_{\text{total}} = P_{H_2} + P_{Cl_2} = 100 \, \text{mm} + 200 \, \text{mm} = 300 \, \text{mm} \]

4. **Conclusion**:
- Both the assertion and the reason are true.
- The assertion correctly reflects the application of Dalton's law, and the reason accurately describes the principle behind the assertion.
- Therefore, we can conclude that both the assertion and the reason are correct, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.