Final answer:
The statement that is part of the cell theory is that 'All living organisms are composed of cells', which is a foundational concept in biology. The cell theory also states that cells are the basic units of life and that cells originate from pre-existing cells through division.
Step-by-step explanation:
Among the options provided, the statement that is part of the cell theory is 'All living organisms are composed of cells'. This important concept is a fundamental principle of biology, affirming that cells are the basic structural and functional units of all living organisms. Additionally, another core aspect of the cell theory is that 'Cells can only arise from pre-existing cells', which indicates that cells multiply through cell division.
The cell theory emphasizes three major points: First, that all organisms are made of one or more cells; second, that cells are the basic unit of life; and third, that all cells arise from pre-existing cells, meaning life continues through a cycle of cell division.
It should be noted that not all cells have a nucleus. For example, prokaryotic cells such as bacteria do not contain a nucleus, while eukaryotic cells, which include animal and plant cells, do. Similarly, statements suggesting that only animal cells are visible under a microscope or that cells depend on their surroundings to provide energy, though partly accurate, are not fundamental principles included in the classic formulation of the cell theory.