A : Magnitude of mean velocity of the gas molecules is same as their mean speed.
R : The only difference between mean velocity and mean speed is that mean velocity is a vector and mean speed is a scalar.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then Mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the question, we need to analyze the assertion (A) and the reason (R) provided:

**Assertion (A):** The magnitude of mean velocity of the gas molecules is the same as their mean speed.

**Reason (R):** The only difference between mean velocity and mean speed is that mean velocity is a vector and mean speed is a scalar.

### Step-by-Step Solution:

1. **Understanding Mean Velocity and Mean Speed:**
- Mean velocity is defined as the vector sum of the velocities of all gas molecules divided by the number of molecules. It has both magnitude and direction.
- Mean speed is defined as the total distance traveled by the gas molecules divided by the total time taken. It is a scalar quantity and does not have a direction.

2. **Analyzing the Assertion (A):**
- The assertion states that the magnitude of mean velocity is the same as mean speed. However, due to the random motion of gas molecules, the vector sum of their velocities can cancel out, leading to a mean velocity of zero.
- In contrast, mean speed, which considers only the magnitude of velocities and not their directions, will be a non-zero value. Thus, the assertion is **incorrect**.

3. **Analyzing the Reason (R):**
- The reason correctly states that mean velocity is a vector and mean speed is a scalar. This is true; however, it does not address the key difference in their magnitudes.
- The reason does not explain why the assertion is incorrect, so while the statement about their nature (vector vs scalar) is correct, it does not justify the assertion. Therefore, the reason is **partially correct** but does not fully support the assertion.

4. **Conclusion:**
- Since the assertion is incorrect and the reason, while correct in its definition, does not support the assertion adequately, both the assertion and reason are wrong.

Thus, the final answer is that both the assertion (A) and the reason (R) are incorrect.

### Final Answer:
Both assertion (A) and reason (R) are incorrect.