To determine which substance has the weakest bonds between its molecules, we can look at the melting points provided. Generally, substances with lower melting points have weaker intermolecular forces.
- Castor oil: -9.8 ºC
- Phenol: 43.2 ºC
- Benzene: 5.7 ºC
- Formic acid: 8 ºC
Among these substances, castor oil has the lowest melting point (-9.8 ºC), which suggests that it has the weakest bonds between its molecules.
Therefore, the answer is castor oil.