The correct answer is C. Cell theory.
Cell theory is the most fundamental generalization in biology. It states that all living things are made up of cells, that cells are the basic units of life, and that all cells come from preexisting cells.
Microbiology is the study of microorganisms, which are organisms that are too small to be seen with the naked eye. Microscopes are instruments that are used to magnify objects, and they were essential for the discovery of cells. The discovery of cells was a major breakthrough in biology, and it led to the development of cell theory.
However, cell theory is more fundamental than microbiology or the discovery of cells. It is a general principle that applies to all living things, while microbiology and the discovery of cells are specific topics that are related to cell theory.