Final answer:
A nonpolar covalent bond has the smallest dipole moment because the electrons are shared equally between the atoms, resulting in no significant charge separation. Option 4 is correct.
Step-by-step explanation:
The bond with the smallest dipole moment is the nonpolar covalent bond. In nonpolar covalent bonds, the electrons are shared equally between two atoms due to very similar or equal electronegativity values, which means that there is no considerable charge separation in the bond.
This differs significantly from polar covalent bonds and ionic bonds where the electronegativity difference causes a displacement of electron density, thus creating a dipole moment.
In the case of polar covalent bonds, this occurs because one atom attracts the bonding electrons more strongly than the other, and for ionic bonds, one atom effectively donates its electron to another, creating charged ions.
By contrast, ionic bonds have the greatest dipole moment because they involve complete transfer of electrons from one atom to another, resulting in two oppositely charged ions.
Polar covalent bonds also have a significant dipole moment, but it is not as strong as that of ionic bonds. Therefore, out of the options provided, the nonpolar covalent bond ranks as the bond with the smallest dipole moment.