To determine which substance has the weakest bonds between its molecules, we generally consider the melting points of the substances. A lower melting point typically indicates weaker intermolecular forces.
Given the melting points:
- Castor oil: –9.8 ºC
- Benzene: 5.7 ºC
- Phenol: 43.2 ºC
- Formic acid: 8 ºC
Castor oil has the lowest melting point at –9.8 ºC, suggesting that it has the weakest intermolecular forces among the substances listed.
So the answer is castor oil.