[A]: Heterosis is superiority of `F_1` hybrid over its two genetically dissimilar parents.
[R]: Heterosis can be measured in terms of size, yield and growth rate

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Heterosis**:
- Heterosis, also known as hybrid vigor, refers to the phenomenon where the `F_1` hybrid offspring exhibit superior qualities compared to their two genetically dissimilar parent strains. This superiority can manifest in various traits such as size, yield, growth rate, and fertility.

2. **Analyzing the Assertion**:
- The assertion states that "heterosis is the superiority of `F_1` hybrid over its two genetically dissimilar parents." This statement is true because it accurately describes the concept of heterosis.

3. **Analyzing the Reason**:
- The reason provided is that "heterosis can be measured in terms of size, yield, and growth rate." This statement is also true as these are common metrics used to evaluate the advantages of hybrids over their parents.

4. **Determining the Relationship**:
- While both the assertion and the reason are true, the reason does not serve as a correct explanation for the assertion. The assertion defines what heterosis is, while the reason provides examples of how heterosis can be measured, but it does not explain why heterosis occurs.

5. **Conclusion**:
- Since both the assertion and reason are true, but the reason does not correctly explain the assertion, the correct answer is that both statements are true, but the reason is not the correct explanation of the assertion.

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

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