Answer: The correct answer for the blank is - 
Cell is the basic structural and functional unit of all life forms, all living organisms are composed of basic unit called cells, and each cell arise from a previously existing cell. 
Explanation - 
The Cell theory was proposed by Schleiden and Schwann in the year 1839, according to which - 
1) Cell is considered as the basic structural and functional unit of all life forms. 
2) All life forms are made up of cell. 
3) Every cell arise from a cell that already existed before.