Option a is the correct answer. The three-age system, divided into stone, bronze and iron from the collection of Copenhagen Museum, was coined by Christian Jürgensen Thomson, a Danish archaeologist who published an essay on the topic in 1837. He based his system on the typological and chronological studies of the artifacts in the museum, and noticed that stone tools preceded bronze tools, which were later replaced by iron tools. The threeage system is still used today as a relative dating system for European and Middle Eastern prehistory.