Assetion In `alpha`-decay atomic number of daughter nucleus reduces by 2 units from the parent nucleus.
Reason An `alpha` particle carries four units of mass.

Options

A. Both assertin and reason are true and reason is the correct explanation of assertion
B. Both assertion and reason are true but reason is not the correct explanation of assertion
C. Assertion is true but reason is flase
D. Both assertion and reason are false

Explanation

To solve the problem, we need to analyze the assertion and reason provided regarding alpha decay.

### Step-by-Step Solution:

1. **Understanding Alpha Decay**:
- In alpha decay, a parent nucleus emits an alpha particle. An alpha particle consists of 2 protons and 2 neutrons, which is equivalent to a helium nucleus.

2. **Effect on Atomic Number**:
- The atomic number (Z) of the parent nucleus decreases by 2 units because the emitted alpha particle contains 2 protons.
- If we denote the parent nucleus as X with atomic number Z, after alpha decay, the daughter nucleus Y will have an atomic number of Z - 2.

3. **Effect on Mass Number**:
- The mass number (A) of the parent nucleus decreases by 4 units because the alpha particle has a total of 4 nucleons (2 protons and 2 neutrons).
- Thus, the mass number of the daughter nucleus Y will be A - 4.

4. **Conclusion on Assertion**:
- The assertion states that in alpha decay, the atomic number of the daughter nucleus reduces by 2 units from the parent nucleus. This is correct.

5. **Understanding the Reason**:
- The reason states that an alpha particle carries four units of mass. This is also correct, as the alpha particle consists of 4 nucleons (2 protons and 2 neutrons).
- However, the reason does not explain why the atomic number decreases by 2. The decrease in atomic number is due to the loss of 2 protons, not directly because of the mass of the alpha particle.

6. **Final Evaluation**:
- Both the assertion and the reason are true, but the reason does not correctly explain the assertion. Therefore, the correct conclusion is that the assertion is true, and the reason is true but not a correct explanation for the assertion.

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