The correct option is C Silver
The most electrically conductive element is silver, followed by copper and gold. Silver also has the highest thermal conductivity of any element and the highest light reflectance. Because silver tarnishes, it is less desirable for high frequencies because the exterior surface is less conductive.
As to why silver is the best conductor, the answer is that its electrons are freer to move than those of the other elements. This has to do with its valence and crystal structure.