First we should know what an atomic pile is. An atomic pile is a nuclear reactor that uses controlled nuclear fission to generate energy. The most common reactor consists of a large assembly of graphite (an allotropic form of carbon) blocks having rods of uranium metal, which acts as the fuel.
The fuel in atomic pile is uranium since it is a substance capable of undergoing nuclear fission and releases a large amount of energy.
Hence, option (C) is correct.