The correct answer is "They prevent the atoms from forming a linear molecule. "
The lone pair of electrons on the O atom of the H₂O molecule repels the bond pair of electrons in the O-H bond. Thus causing H₂O to acquire a bent structure instead of a liner structure. If H₂O were a linear molecule the dipole of the each O-H bond would cancel out each other. However, in a bent structure each O-H bond contributes to the overall dipole of the H₂O molecule. Thus contributing to the polarity of a water molecule.
