$ In case of human beings, the process of focusing of an object by the eyes is called dioptric part.
! Diatoms are a type of Nektons.

Options

A. If both As and R are correct and R is correct explanation of As
B. If both As and R are correct but R is not correct explanation of As
C. If As is correct, R is wrong
D. If both As and R are wrong.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in human beings, the process of focusing an object by the eyes is called the dioptric part.
- The dioptric part of the eye includes structures such as the cornea and lens, which work together to focus light onto the retina, allowing us to see clearly.

2. **Evaluating the Assertion**:
- The assertion is true. The dioptric part of the eye does play a crucial role in focusing images. The cornea and lens adjust the eye's optical power to maintain a clear image of objects at varying distances.

3. **Understanding the Reason**:
- The reason states that diatoms are a type of nektons.
- Nektons are organisms that can swim independently in water, such as fish and marine mammals.

4. **Evaluating the Reason**:
- This statement is incorrect. Diatoms are single-celled algae that are part of the phytoplankton community and do not swim independently; they are mostly carried by water currents. Therefore, they do not fit the definition of nektons.

5. **Conclusion**:
- The assertion is true, while the reason is false. Thus, the correct answer is that the assertion is correct, and the reason is incorrect.

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

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