Answer:
Magnesium is known for a long time as the lighter structural metal in the industry, due to it's low weight and to it's capability of forming mechanically resistant alloys.
Following steps are involved in this reaction,
Nitration: Nitration of benzene yield nitrobenzene.
Reduction: Reduction of Nitrobenzene yield Aniline.
Diazotization: Diazonium alt of benzene is formed from aniline.
At last diazonium salt is reacted with CuCl to yield chlorobenzene.