Unsaturated fats are fats that have one or more carbon-carbon double bond in their chain. The double bond causes the molecule to bend. The molecules do not stack well, which causes the intermolecular forces to be much weaker than those of saturated fats. This causes the melting points to be lower for unsaturated fats.
Saturated fats have no double bonds, only single bonds between their carbons. This fact enables them to "stack" efficiently and pack tightly, which means the molecule has a high melting point and remains solid even at a relatively high temperature. Butter or bacon fat will definitely solidify in the fridge, and often stay solid outside the fridge.
Therefore unsaturated fats have double bonds (eliminate A and C) and a lower melting point (eliminate D)
