Among the following species, the diamagnetic molecule is

A carbon monoxide molecule is diamagnetic because every electron in it is paired.

Its electronic configuration can be expressed as
$\mathrm{\sigma }1{\mathrm{s}}^2,{\mathrm{\sigma }}^{\ast }1{\mathrm{s}}^2,\mathrm{\sigma }2{\mathrm{s}}^2,{\mathrm{\sigma }}^{\ast }2{\mathrm{s}}^2,\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}^2, \left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}^2=\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}^2\right)$.

${\mathrm{O}}_2,\mathrm{NO},{\mathrm{B}}_2$ are paramagnetic according to M.O.T. where as $\mathrm{CO}$ is diamagnetic.

Hence, the correct option is  (C).