What is the periodic structure called in which layers of different materials, each only a few nm thick, are stacked alternately?

Answer: C. Superlattice

A superlattice periodically stacks extremely thin layers of different materials, such as GaAs and AlGaAs. It shows electronic and optical properties not found in the original materials, so it is used in optical devices such as quantum cascade lasers and as buffer layers to reduce dislocations.