d
The \(CO\) donates two electrons and \(NO\) donates three electrons.

So, \(\left(3 \times 2 e ^{-}=6 e ^{-}\right) ; 2\) \(NO\) can easily replaced \(3 CO\).

\(\left(6 \times 2 e ^{-}=12 e ^{-}\right) ; 4 NO\) can easily replace \(6 CO\)

\(Fe ( CO )_{5} \rightarrow Fe ( NO )_{2}( CO )_{2}\)

\(Cr ( CO )_{5} \rightarrow Cr ( NO )_{4}\)