Option C is correct because prokaryotic cells are characterized by the absence of membrane-bound organelles. This is a fundamental distinction between prokaryotic and eukaryotic cells. Option A is incorrect because not all living organisms have cells with a nucleus - prokaryotes lack a well-defined nucleus. Option B is incorrect because while plant cells have a well-defined cell wall, animal cells do not possess a cell wall. Option D is incorrect because according to the cell theory mentioned in the context, Virchow explained that cells are formed from pre-existing cells, not de novo from abiotic materials. Question 2 in the exercises also confirms that new cells generate from pre-existing cells.