Final answer:
The intermolecular force that must be strongest for an ionic substance to dissolve in water is the ion-dipole attraction.
Step-by-step explanation:
In order for an ionic substance to dissolve in water, the intermolecular force that must be strongest is the ion-dipole attraction. When an ionic substance dissolves in water, the water molecules surround the individual ions and form a solvation shell, stabilizing the ions and preventing them from recombining. This is due to the strong attraction between the positive and negative charges of the ions and the partial charges on the water molecules.