Which of the following is not correct?
(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 given statements is not correct, we will analyze each option one by one.

### Step-by-Step Solution:

1. **Analyze Statement (a)**: "Robert Brown discovered the cell."
- Robert Brown is known for discovering the nucleus within the cell, not the cell itself. The cell was actually discovered by Robert Hooke in 1665. Therefore, this statement is incorrect.

2. **Analyze Statement (b)**: "Schleiden and Schwann formulated the cell theory."
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