Assertion : At `F_(2)` stage in monohybrid cross, both parental traits are expressed in the proportion of `3:1`
Reason : The contrasting parental traits show blending at `F_(2)` stage

Options

A. If both the assertion and the reason are true and the reason is a correct explanation of the assertion.
B. If both the assertion and the reason are true but the reason is not a correct explanation of the assertion.
C. If the assertion is true but the reason is false.
D. If both the assertion and reason are false.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that at the F2 stage of a monohybrid cross, both parental traits are expressed in the ratio of 3:1. This is a fundamental concept in Mendelian genetics.

2. **Monohybrid Cross Explanation**: In a monohybrid cross, we consider one trait that has two contrasting forms (e.g., tall and dwarf plants). The parental generation (P) consists of one homozygous tall plant (TT) and one homozygous dwarf plant (tt).

3. **F1 Generation**: When these two parents are crossed, all offspring in the F1 generation will be heterozygous (Tt) and will express the dominant trait, which is tall in this case.

4. **F2 Generation**: When the F1 generation (Tt) is self-fertilized, the F2 generation will have the following genotype combinations:
- TT (homozygous tall)
- Tt (heterozygous tall)
- Tt (heterozygous tall)
- tt (homozygous dwarf)

5. **Phenotypic Ratio**: The phenotypic ratio of the F2 generation will be:
- 3 tall (TT and Tt) : 1 dwarf (tt)
This confirms the assertion that both parental traits are expressed in the ratio of 3:1.

6. **Understanding the Reason**: The reason states that the contrasting parental traits show blending at the F2 stage. This is incorrect in the context of a monohybrid cross.

7. **Blending Inheritance**: Blending inheritance would imply that traits mix together to form an intermediate phenotype. In a monohybrid cross, the dominant trait does not blend with the recessive trait; instead, it masks it. Therefore, the F2 generation does not exhibit blending but rather maintains the distinct traits.

8. **Conclusion**: The assertion is true (3:1 ratio at F2), but the reason is false (no blending occurs). Thus, the correct answer is that the assertion is true, but the reason is false.

### Final Answer:
- **Assertion**: True
- **Reason**: False