Mark out the correct statement regarding X-rays.

Options

A. When fast moving electrons strike the metal target, they enter the metal larged and in a very short time span come to rest , and thus an electromagnetic charged electron produces electromagnetic waves (X-ray).
B. Characteristic X-rays are produced due to transition of an electron from higher energy levels to vacant lower energy levels.
C. X-ray spectrum is a discrete spectra just like hydrogen spectra.
D. Both (a) and (b) are correct.

Explanation

To solve the question regarding the correct statement about X-rays, we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Analyze Statement A**:
- Statement A claims that when fast-moving electrons strike a metal target, they come to rest quickly, producing X-rays.
- This statement is partially correct because it describes the general process of X-ray production (bremsstrahlung radiation) but does not account for the production of characteristic X-rays, which occur due to electron transitions between energy levels in the atom.
- **Conclusion**: Statement A is incorrect.

**Hint**: Consider the different mechanisms of X-ray production: bremsstrahlung vs characteristic.

2. **Analyze Statement B**:
- Statement B states that characteristic X-rays are produced due to the transition of an electron from a higher energy level to a vacant lower energy level.
- This statement is correct because characteristic X-rays are indeed produced when an electron falls to a lower energy level after an inner-shell electron has been ejected.
- **Conclusion**: Statement B is correct.

**Hint**: Recall how characteristic X-rays are formed in terms of electron transitions.

3. **Analyze Statement C**:
- Statement C asserts that the X-ray spectrum is a discrete spectrum, similar to the hydrogen spectrum.
- This statement is incorrect because while characteristic X-rays produce discrete lines (peaks) in the spectrum, the overall X-ray spectrum includes a continuous range of wavelengths (bremsstrahlung radiation).
- **Conclusion**: Statement C is incorrect.

**Hint**: Differentiate between continuous and discrete spectra in X-ray production.

4. **Analyze Statement D**:
- Statement D claims that both A and B are correct.
- Since we have established that Statement A is incorrect and Statement B is correct, this statement must also be incorrect.
- **Conclusion**: Statement D is incorrect.

**Hint**: Evaluate the truth of each statement before concluding on combined statements.

### Final Answer:
The only correct statement regarding X-rays is **Statement B**.