Which of the following is a concept of the cell theory?

Cell theory states that living things are composed of one or more cells, that the cell is the basic unit of life, and that cells arise from existing cells.

How did scientists understanding of cells develop?

With the invention of the microscope by Zacharias Janssen, scientists found a new way to investigate the world. Robert Hooke discovered cells while looking at a piece of cork through a microscope and Anton van Leeuwenhoek observed the first living cells. This paved the way for modern cellular science.