A Nucleus was discovered by Robert Brown
R Large sized organisms have large sized cells.

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "A nucleus was discovered by Robert Brown."
- Robert Brown, a Scottish botanist, indeed discovered the nucleus while studying orchids. He observed a distinct area within the cell, which he termed the nucleus.

2. **Understanding the Reason**:
- The reason states that "Large sized organisms have large sized cells."
- This statement is misleading. While large organisms do have larger overall sizes, it is not necessarily true that their cells are larger in size. Instead, large organisms typically consist of a greater number of cells, which collectively contribute to their larger size.

3. **Evaluating the Assertion and Reason**:
- The assertion is correct because Robert Brown did discover the nucleus.
- The reason is incorrect because large organisms do not have larger cells; they have more cells of the same size.

4. **Conclusion**:
- Based on the evaluation, the correct answer is that the assertion is correct, but the reason is incorrect. Therefore, the answer is option C: Assertion is correct and Reason is wrong.