Answer: The correct answer for the blank is -
All three options.
Explanation -
The Cell theory was proposed by Schleiden and Schwann in 1839, which follows-
1) Cell is considered as the basic structural and functional unit of all living organisms.
2) All life forms are composed of cell.
3) Every cell comes or arises from a cell that already existed before.
Thus, all three options are correct in this question.