Answer: The correct option is d. the improvement of the microscope and microscopic techniques.
In the 1600s the development of cell theory was led by advances in microscopy. The cell theory is historic in terms of scientific theory. According to the theory it was proposed that all living organisms are made up from cells, cell is the basic structural unit in all the organisms and new cells arise from pre-existing cells. The continuous improvement of microscopy with time and advancements in magnification technology were enough for discovering the cells and contributed the most towards the cell theory.
