(A) Electronic configuration of Cu is 1s22s22p63s23p63d104s1. ΔiH1 is low while ΔiH2 is highest because of fully filled d-orbital. (B) Electronic configuration of Zn is 1s22s22p63s23p63d104s2 First 2 electrons are removed from s-orbital while 3rd electron released from fully filled d-orbital, which require large amount of energy. (C) M in [M(CO)6] is Cr. (D) Ni has highest heat of atomisation in the 3d-transition series.