Assertion : Soft and hard `X`-rays differ in frequency as well as velocity.
Reason : The penetrating power of hard `X`- rays is more than the penetrating power of soft `X`-rays.

Options

A. If both Assertion `&` Reason are True `&` the Reason is a correct explanation of the Assertion.
B. If both Assertion `&` Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion `&` Reason are False.

Answer

To solve the question regarding the assertion and reason about soft and hard X-rays, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Soft and hard X-rays differ in frequency as well as velocity."
- X-rays are electromagnetic waves, and their properties are determined by their frequency and wavelength.
- Hard X-rays have a higher frequency and energy compared to soft X-rays.
- However, all electromagnetic waves, including X-rays, travel at the same speed in a vacuum, which is the speed of light (approximately \(3 \times 10^8 \, \text{m/s}\)).
- Therefore, the assertion is **false** because while they differ in frequency, they do not differ in velocity.

2. **Understanding the Reason**:
- The reason states that "The penetrating power of hard X-rays is more than the penetrating power of soft X-rays."
- Hard X-rays have higher energy and shorter wavelengths, which allows them to penetrate materials more effectively than soft X-rays.
- This statement is **true** because hard X-rays can penetrate deeper into materials and tissues compared to soft X-rays.

3. **Conclusion**:
- Since the assertion is false and the reason is true, the correct answer is that the assertion is false and the reason is true.

### Final Answer:
- The assertion is false, and the reason is true. Therefore, the correct option is D: Assertion is false and reason is true.