What causes a molecule to have a net dipole moment?
A. The presence of only nonpolar bonds in the molecule
B. The presence of Van der Waals forces
C. The presence of a net charge that does not cancel out
D. The presence of both covalent and ionic bonds in the molecule

Option C

Explanation:

Dipole moment being ac vector quantity has both magnitude and direction. All bonding forces have dipole moment ranging from partially positive to negative moment.  

Net dipole moment is the dipole moment arising after summation of all the existing dipole moments. On the basis of dipole moment it is determined whether the molecule is polar or non polar.  

Hence, option c is correct