Answer:
A. Microscope
Step-by-step explanation:
If you look at the definition of each thing that you listed, you'll be able to deduct which is the correct answer.
Microscope - an optical instrument used for viewing very small objects, such as mineral samples or animal or plant cells, typically magnified several hundred times.
Telescope - an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified.
Nucleus - the central and most important part of an object, movement, or group, forming the basis for its activity and growth.