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 following statements is not correct, we will analyze each option one by one:

1. **Statement (a): Robert Brown discovered the cell.**
- This statement is incorrect. Robert Brown did not discover the cell; he is known for discovering the nucleus within the cell. The term "cell" was first coined by Robert Hooke in 1665 when he observed cork under a microscope.

2. **Statement (b): Schleiden and Schwann formulated the cell theory.**
- This statement is correct. Matthias Schleiden and Theodor Schwann are credited with formulating the cell theory in the 19th century, which states that all living organisms are composed of cells, and that the cell is the basic unit of life.

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