The correct answer is C, living things are made of cells, which was first discovered using a microscope. In 1665, Robert Hooke used a microscope to observe thin slices of cork, and he saw tiny boxes or compartments that he called "cells" because they reminded him of the small rooms or cells that monks lived in. Hooke's discovery paved the way for the development of the cell theory, which states that all living organisms are composed of one or more cells.