Answer: The correct answer for the fill in the blank is None of the above.
Schleiden and Schwann proposed the Cell theory in 1838. According to this theory-
- One or more cell constitute the living organisms.
- The cell is considered as the basic unit of organization and structure in all living forms.
- All cells have originated from the already existing cells.
Thus, according to the cell theory, cell is the only basic unit of life. This is not mentioned in any of the the options. Therefore, the answer is none of the above.