The cell theory states that _________A. All cells have nuclei.B. All cells are living.C. Cells divide by both mitosis and meiosis.D. The cell is the functional and structural unit of plants and animals.

The correct answer is:

D. The cell is the functional and structural unit of plants and animals.

Explanation:

The cell theory is a fundamental concept in biology that states:

All living organisms are composed of cells.
The cell is the basic unit of structure and function in living organisms.
All cells arise from pre-existing cells.
Option D aligns with this idea because it emphasizes that the cell is both the functional and structural unit of living organisms, including plants and animals.

The other options are incorrect for the following reasons:

A. All cells have nuclei: This is not true, as some cells (like prokaryotes) do not have a nucleus.
B. All cells are living: While most cells are living, some (like red blood cells in humans) are considered non-living once they lose their nuclei or undergo certain changes.
C. Cells divide by both mitosis and meiosis: This is true for certain types of cells, but not a part of the general cell theory itself.