Potential car buyers working on car company can make offer on multiple cars that are listed with the car Company. Offer amount can be modified; however the car that has the offer cannot be modified after the offer is placed?

What should be done to associated offer with car in the schema for organization?

1 - create lookup with offer to car custom object
2-  create master detail with offer to car custom object
3-  create lookup with car to offer custom object
4-  create master detail in contact object to both car and offer custom object.