The correct answer is: C. Electron microscopes.
Electron microscopes are often used to view metal surfaces because they can magnify objects up to 100 million times, which is much more than optical microscopes can. They can also see objects that are much smaller than the wavelength of light, which is why they are often used to study the structure of materials.
Optical microscopes use light to magnify objects. They can magnify objects up to 2,000 times, but they cannot see objects that are smaller than the wavelength of light.
Scanning tunneling microscopes use a very small needle to scan the surface of an object. They can magnify objects up to 100 million times, but they can only see objects that are on the surface of a material.
Therefore, electron microscopes are the best type of microscope to use to view metal surfaces.