Assertion :- According to Mendel, factors are stable & discrete units that control the expression or traits.
Reason :- He could not provide any physical proof for the existance of factors.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that according to Mendel, factors are stable and discrete units that control the expression of traits. This is true because Mendel's work laid the foundation for genetics, and he referred to these units as "factors," which we now know to be genes.

2. **Understanding the Reason**: The reason given is that Mendel could not provide any physical proof for the existence of these factors. This is also true; Mendel conducted his experiments and made observations based on ratios and patterns in inheritance but did not have the technology or knowledge to identify the physical structure of genes (DNA).

3. **Evaluating the Relationship**: While both the assertion and the reason are true, the reason does not correctly explain the assertion. The assertion is about the nature of factors (genes) as stable and discrete units, while the reason discusses Mendel's inability to provide physical proof. The lack of physical proof does not negate the validity of the assertion regarding the nature of factors.

4. **Conclusion**: Since both the assertion and the reason are true, but the reason is not a correct explanation of the assertion, we can conclude that the correct option is that both statements are true, but the reason is not the correct explanation of the assertion.

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