Electron microscopy was introduced by:: (A) Leeuwenhoek, (B) Robert Koch, (C) Ruska, (D) Paul Ehrlich
Answer is C. The first prototype electron microscope, capable of four-hundred-power magnification, was developed in 1931 by the physicist Ernst Ruska and the electrical engineer Max Knoll. 
The apparatus was the first practical demonstration of the principles of electron microscopy.