Cell theory is considered a scientific theory in biology primarily for the following reason:
D. There is lots of evidence to support it.
Cell theory is supported by extensive empirical evidence gathered over many years, demonstrating that all living organisms are composed of cells, that cells are the basic unit of life, and that all cells arise from pre-existing cells. This theoretical framework is further strengthened by consistent observations and experiments across various biological disciplines. While options B and C have merit, the foundational basis of a scientific theory is the substantial evidence that backs it, which option D directly addresses.