The correct answer is : (iii)
Experiments that showed that all cells are similar in how they metabolize and in their composition.
The cell theory is really an idea that came about regarding cells and living organisms. The 1st person to discover and name cells was Mr. Robert Hooke, who coined the term to describe the structures he saw making up the cork.
This would not have been possible if it had not been for the development of the microscope during the 17th century, which allowed these cells to be visible.