Answer:
C. Specimens are placed in a vacuum is the correct answer.
Step-by-step explanation:
The electron microscope uses a beam of an electron instead of the light that's the reason vacuum is needed. If the specimens are not placed in a vacuum, electrons will hit the air particles instead of a specimen.
To form a image the electrons are needed to make contact with specimen and living tissue is not able survive in the electron microscopy,as the for the preparation of tissue chemicals are used which kill the living tissue and they also not survive in the vacuum that's the reason in the transmission electron microscopes dead specimens is used and the Specimens are placed in a vacuum.