Answer:
D) voidable.
Explanation:
A voidable contract is a contract that provides good title until it is voided by one of the parties. In this case, the boy is not allowed to engage in a real estate contract, but if he insists he can go ahead with it. The buyer is at risk here because the boy can void the contract in the future based on the fact that he was a minor when it was made.
This can get really messy if this buyer then sells the property to another good faith buyer, since that title would be considered good and valid (if the contract hasn't been voided yet) and then the boy would have to sue the original buyer and claim compensation.