Final answer:
Property damage liability coverage of the auto policy would pay for the damage to the store.
Step-by-step explanation:
The part of the insured's auto policy that would pay for the damage to the store is Property damage liability.
Property damage liability coverage is a type of car insurance that helps cover the costs of damage that you may cause to another person's property, such as a store front, in an accident.
In this case, since the trailer came loose from the insured's auto and ran into the store front, the property damage liability coverage of the auto policy would pay for the damage to the store.