# The common features among the species $CN^-,CO$ and $NO^+$ are :

$\begin{array}{1 1}(a)\;\text{bond order three and isoelectronic}\\(b)\; \text{isoelectronic and weak field ligands}\\(c)\;\text{bond order two and }\pi-\text{acceptors}\\(d)\;\text{bond order three and weak field ligands}\end{array}$

According to Molecular Orbital theory,
$CN^-,CO$ and $NO^+$ are isoelectronic $(14e^-)$ and bond order is 3.
Hence (a) is the correct answer.
edited Apr 10