Which of the following is not correct?

Options

A. Robert Brown discovered the cell
B. Schleiden and Schwann formulated the cell theory
C. Virchow explained that cells are formed from pre-existing cells
D. A unicellular organism carries out its life activities within a single cell

Explanation

To determine which of the following statements is not correct, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Option 1: Robert Brown discovered the cell.**
- Robert Brown is known for discovering the nucleus of the cell, not the cell itself. The cell was discovered by Robert Hooke. Therefore, this statement is **not correct**.

2. **Option 2: Sladen and Swan formulated the cell theory.**
- This statement is correct. The cell theory was indeed formulated by Schleiden and Schwann (note the correct spelling). They proposed that all living things are composed of cells, and that the cell is the basic unit of life. Thus, this statement is **correct**.

3. **Option 3: Virchow explained that cells are formed from pre-existing cells.**
- This statement is also correct. Rudolf Virchow proposed the idea that all cells arise from pre-existing cells, which is a fundamental concept in cell biology. Therefore, this statement is **correct**.

4. **Option 4: Unicellular organisms carry out their life activities within a single cell.**
- This statement is correct as well. Unicellular organisms, such as bacteria and protozoa, perform all necessary life functions within one cell and can exist independently. Thus, this statement is **correct**.

### Conclusion:
The only statement that is not correct is **Option 1**: "Robert Brown discovered the cell."