Assertion : After emission of one `alpha`-particle and two `beta`-particles, atomic number remains unchanged.
Reason : Mass number changes by four.

Options

A. (a) If both Assertion and Reason are true and Reason is correct explanation of the Assertion.
B. (b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Explanation

To solve the question, we need to analyze both the assertion and the reason given in the context of nuclear decay processes, specifically alpha and beta decay.

### Step-by-Step Solution:

1. **Understanding Alpha Decay**:
- In alpha decay, an atomic nucleus emits an alpha particle (which consists of 2 protons and 2 neutrons).
- As a result, the atomic number (Z) decreases by 2 and the mass number (A) decreases by 4.
- For example, if we start with a nucleus of element A with atomic number Z and mass number A, after alpha decay, it will transform into a new element with atomic number \( Z-2 \) and mass number \( A-4 \).

2. **Understanding Beta Decay**:
- In beta decay, a neutron in the nucleus is transformed into a proton, emitting a beta particle (an electron).
- This process increases the atomic number (Z) by 1, while the mass number (A) remains unchanged.
- Therefore, if we have a nucleus with atomic number Z and mass number A, after beta decay, it will become a new element with atomic number \( Z+1 \) and mass number A.

3. **Combining Alpha and Beta Decay**:
- The question states that one alpha particle and two beta particles are emitted.
- After the emission of one alpha particle, the atomic number changes from Z to \( Z-2 \) and the mass number changes from A to \( A-4 \).
- Then, after the emission of two beta particles, the atomic number changes from \( Z-2 \) to \( Z-2+2 \) (i.e., it increases by 2) and the mass number remains \( A-4 \).

4. **Final Atomic Number and Mass Number**:
- After one alpha decay and two beta decays, the final atomic number will be:
\[ Z' = Z - 2 + 2 = Z \]
- The final mass number will be:
\[ A' = A - 4 \]
- Thus, the atomic number remains unchanged, while the mass number decreases by 4.

5. **Evaluating the Assertion and Reason**:
- **Assertion**: After emission of one alpha particle and two beta particles, the atomic number remains unchanged. (True)
- **Reason**: Mass number changes by four. (True)
- However, the reason provided does not explain the assertion correctly, as the assertion is about atomic number while the reason discusses mass number.

### Conclusion:
Both the assertion and the reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Both assertion and reason are true, but the reason is not the correct explanation of the assertion.