Final answer:
Once a molecule is found to contain polar bonds, for the whole molecule to be polar, it must have a net dipole moment because this indicates that the polar bond dipoles do not cancel each other out due to an asymmetrical molecular structure. The correct option is (C).
Step-by-step explanation:
To determine if a molecule with polar bonds is overall polar, we need to consider its molecular geometry and how the individual bond dipole moments interact. For the whole molecule to be polar, it must have a net dipole moment. The individual dipoles of the polar bonds must not cancel each other out; this typically occurs in molecules with an asymmetrical shape. So, even though a molecule contains polar bonds, if its structure is such that the bond polarities are symmetrically arranged and cancel each other out, the molecule will be nonpolar.
In contrast, a molecule with asymmetrical distribution of polar bonds will usually be polar because the dipoles do not cancel out. For example, water (H2O) is a polar molecule because it has a bent structure, leading to an uneven distribution of electrical charge and a net dipole moment. In short, for the molecule to be polar, the key is the asymmetrical arrangement of the polar bonds resulting in a net dipole.
Based on the options provided in the question, the correct option for a molecule to be polar, once it's found to contain polar bonds, is C) The molecule must have a net dipole moment.