The correct answer is: a) all cells have a nucleus.
Cell theory is a foundational principle in biology that describes the properties of cells. It includes three main points: 1) All living organisms are composed of one or more cells; 2) The cell is the basic unit of life; and 3) All cells arise from preexisting cells.
The statement that ‘all cells have a nucleus’ is incorrect because while many cells, such as plant and animal cells, do have a nucleus, there are also many types of cells, like bacterial cells, that lack a nucleus. Bacteria are classified as prokaryotic cells, which do not have a membrane-bound nucleus, thus not all cells fitting within the framework of cell theory possess a nucleus.