Which of the following molecules has a dipole moment of zero?

The chemical formula for ${\mathrm{CO}}_2$ is $\mathrm{O}=\mathrm{C}=\mathrm{O}$, and it has a bond dipole moment ($\mathrm{C}-\mathrm{O}$ bond). However, because one bond's dipole moment is negated by another, ${\mathrm{CO}}_2$ has a linear structure. As a result of being a linear molecule, ${\mathrm{CO}}_2$ has no dipole moment.