If the given molecule/atom/ion has all paired electrons in its orbitals, then it is said to be diamagnetic and if atleast one of the orbital has unpaired electron, then it is said to be paramagnetic.

Writing the molecular orbital configuration of the given molecules assuming that no $2\mathrm{s}-2\mathrm{p}$ mixing takes place-

(i) $\mathrm{B}{\mathrm{e}}_2\to \mathrm{\sigma }1{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}1{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}2{\mathrm{s}}^2$ (diamagnetic)

(ii) ${\mathrm{B}}_2\to \mathrm{\sigma }1{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}1{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}2{\mathrm{s}}^2,\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}^2$ (diamagnetic)

(iii) ${\mathrm{C}}_2\to \mathrm{\sigma }1{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}1{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}2{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}^2,\begin{array}{c}\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}^1\\ \mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}^1\end{array}$ (paramagnetic)

(iv) ${\mathrm{N}}_2\to \mathrm{\sigma }1{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}1{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{s}}^2,\mathrm{ }{\mathrm{\sigma }}^{\mathrm{*}}2{\mathrm{s}}^2,\mathrm{ }\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}^2,\mathrm{ }\begin{array}{c}\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}^2\\ \mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}^2\end{array}$ (diamagnetic)

Note here that, if any of the molecular orbital has two electrons, then the electrons will be paired and if it has one electron, then the electron will be unpaired.