How many among ${\mathrm{C}}_6{\mathrm{H}}_6,\mathrm{CO},{\mathrm{SO}}_2$ and ${\mathrm{NH}}_3$ has/have zero dipole moment?

Which of the following statement is incorrect from the view point of molecular orbital theory:
a) ${\mathrm{Be}}_2$ is not a stable molecule.
b) ${\mathrm{He}}_2$ is not stable but ${\mathrm{He}}_2^{+}$ is expected to exist.
c) Bond strength of ${\mathrm{N}}_2$ is maximum amongst the homo nuclear diatomic molecules belonging to the second period.
d) The order of energies of molecular orbitals in ${\mathrm{N}}_2$ molecule is:

$\mathrm{\sigma }2\mathrm{s}<{\mathrm{\sigma }}^{*}2\mathrm{s}<\mathrm{\sigma }2{\mathrm{p}}_{\mathrm{z}}<\left(\mathrm{\pi }2{\mathrm{p}}_{\mathrm{x}}=\mathrm{\pi }2{\mathrm{p}}_{\mathrm{y}}\right)<\left({\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{x}}={\mathrm{\pi }}^{*}2{\mathrm{p}}_{\mathrm{y}}\right)<{\mathrm{\sigma }}^{*}2{\mathrm{p}}_{\mathrm{z}}$