Assertion: Under similar conditions of temperature and pressure,`O_(2)` diffuses `1.4` times faster than `SO_(2)`.
Reason: Density of `SO_(2)` is `1.4` times greater than that of `O_(2)`.

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 both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that under similar conditions of temperature and pressure, \( O_2 \) diffuses 1.4 times faster than \( SO_2 \).
- According to Graham's Law of Effusion/Diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.

2. **Applying Graham's Law**:
- Graham's Law can be expressed as:
\[ \frac{R_1}{R_2} = \sqrt{\frac{M_2}{M_1}} \]
- Here, \( R_1 \) and \( R_2 \) are the rates of diffusion of \( O_2 \) and \( SO_2 \) respectively, and \( M_1 \) and \( M_2 \) are their molar masses.
- The molar mass of \( O_2 \) is approximately 32 g/mol, and the molar mass of \( SO_2 \) is approximately 64 g/mol.

3. **Calculating the Rate of Diffusion**:
- Plugging in the values:
\[ \frac{R_{O_2}}{R_{SO_2}} = \sqrt{\frac{M_{SO_2}}{M_{O_2}}} = \sqrt{\frac{64}{32}} = \sqrt{2} \approx 1.41 \]
- This means \( O_2 \) diffuses approximately 1.41 times faster than \( SO_2 \), which supports the assertion.

4. **Understanding the Reason**:
- The reason states that the density of \( SO_2 \) is 1.4 times greater than that of \( O_2 \).
- Density is directly related to molar mass, as density \( \rho \) can be expressed as:
\[ \rho = \frac{m}{V} \]
- Since \( SO_2 \) has a higher molar mass than \( O_2 \), it will indeed have a greater density.

5. **Conclusion**:
- Both the assertion and the reason are correct.
- The reason correctly explains why \( O_2 \) diffuses faster than \( SO_2 \) based on their densities and molar masses.

### Final Answer:
Both assertion (A) and reason (R) are correct, and R is the correct explanation of A.