Answer:
a. do nothing.
Explanation:
In this scenario the contract to buy the car was entered by your 15 year old brother. According to the law the legal age for an individual to be able to enter into a legal contract is 18 years and above.
If a person is less than 18 years they will require the guardian to be the person entering the contract on their behalf.
Since the contract was not entered into with the father's knowledge, this is not a legally binding contract.
So the stranger cannot sue in this instance