Hello,

BDC ( Benzene Diazonium Chloride ) is generally prepared by Aniline which is formed from Nitro-Benzene's reduction.

First Step is Nitration of Benzene to form Nitro-Benzene

Second Step Involves Reduction of Nitro-Benzene to Aniline

Third and Final Step is  Formation of Benzene diazonium chloride  in presence of dilute HCl or HNO3

Hope this was helpful