The correct answer is: B. A contract to do or not to do something, if some event, collateral to such contract, does or does not happen.
A contingent contract is a contract that is dependent on the happening or non-happening of a certain event. The event must be uncertain, and it must be outside the control of the parties to the contract. If the event does not happen, then the contract is void.
Option A is incorrect because it describes a wager. A wager is a bet on the outcome of an uncertain event. Wagers are not enforceable by law.
Option C is incorrect because it describes an uncertain agreement. An uncertain agreement is not a contract. A contract must be certain in order to be enforceable.
Option D is incorrect because it describes an unenforceable contract. An unenforceable contract is a contract that is not valid or binding. An unenforceable contract may be void, voidable, or unenforceable.