a
The ionisation enegies increase with increase in atomic number. However, the tread is irregular among some \(d-\) block element. on the basis of electronic configuration , the

\(Zn\,:\,1{s^2}\,2{s^2}\,{p^6}\,3{s^2}\,{p^6}\,{d^{10}}\,4{s^2}\)

\(Fe\,:\,1{s^2}\,2{s^2}\,{p^6}\,3{s^2}\,{p^6}\,{d^6}\,4{s^2}\)

\(Cu\,:\,1{s^2}\,2{s^2}\,{p^6}\,3{s^2}\,{p^6}\,{d^{10}}\,4{s^1}\)

\(Cr\,:\,1{s^2}\,2{s^2}\,{p^6}\,3{s^2}\,{p^6}\,{d^5}\,4{s^1}\)

\(IE_1\) follows the order \(:\,Zn > Fe > Cu > Cr\)