The property that will be least helpful in the determination of the bond type that has the strongest intermolecular forces is C. Conductivity in water.
It should be noted that the students wanted to investigate whether ionic or covalent bonds have stronger intermolecular forces.
In order to carry out the investigation, salt, sugar, and molar balls are used. Based on this, the property that is required for the investigation will be flammability, solubility in water, and the melting point.
The conductivity in water isn't required.