# Which of the following is not isoelectronic?

$\begin{array}{1 1}(a)\;NO^+&(b)\; CO\\(c)\;CN^-&(d)\;O_2^-\end{array}$

No of electrons in $NO^+$ is 14
No of electrons in $CO$ is 14
No of electrons in $CN^-$ is 14
No of electrons in $O_2^-$ is 17
$O_2^-$ is not an isoelectronic with $NO^+,CO$ and $CN^-$
Hence (d) is the correct answer.