The observations of Hooke and van Leeuwenhoek documenting the existence of microscopic cells formed the basis of what important theory?

Cell theory

Explanation:

Robert Hooke modified the compound microscope and observed the microscopic organisms and the rectangular blocks of cork cells. He named the blocks called cells.

Similarly, Anton van Leeuwenhoek observed the living fungus growing on the bread with microscope and observed that these organisms are composed of the same cells.

Later the Schleiden and Schwann observed the plant tissue and animal tissue independently and found that these are made of the cells. This lead to the formation of the cell theory that all living organisms are made of cells.

Thus, cell theory is the correct answer.