Assertion : The apparent frequency will remain same as the frequency of source of sound if source and listener move at right angles to each other.
Reason : Apparent frequency will always increase whether source is moving towards the listener or listener is moving towards the source.

Options

A. If both assertion and reason are correct and reason is a correct explanation of the assertion.
B. If both assertion and reason are correct but reason is not the correct explanation of assertion.
C. If assertion is correct but reason is incorrect
D. If assertion is incorrect but reason is correct.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "The apparent frequency will remain the same as the frequency of the source of sound if the source and listener move at right angles to each other."

- When the source and listener move at right angles, their relative velocity along the line connecting them is zero. This means that there is no approach or recession between them.
- Therefore, the distance between the source and listener is increasing, which implies that the apparent frequency will change.

**Conclusion**: The assertion is **incorrect**.

### Step 2: Analyze the Reason
The reason states that "Apparent frequency will always increase whether the source is moving towards the listener or the listener is moving towards the source."

- According to the Doppler effect, if the source is moving towards the observer or the observer is moving towards the source, the apparent frequency increases.
- The formula for the Doppler effect is given by:
\[ f' = f \frac{v + v_o}{v - v_s} \]
where:
- \( f' \) is the apparent frequency,
- \( f \) is the source frequency,
- \( v \) is the speed of sound,
- \( v_o \) is the speed of the observer (positive if moving towards the source),
- \( v_s \) is the speed of the source (positive if moving away from the observer).

- In both cases (source moving towards the observer or observer moving towards the source), the numerator increases while the denominator decreases, leading to an increase in the apparent frequency.

**Conclusion**: The reason is **correct**.

### Final Conclusion
Since the assertion is incorrect and the reason is correct, the answer to the question is option 4.