In which type of contract are the obligations contingent upon an uncertain future event?

The correct choice is the aleatory contract, which is characterized by obligations that depend on the occurrence of an uncertain future event. In an aleatory contract, the performance of one or both parties is reliant on a specific event or outcome, which may or may not happen. This means that the risk is shared between the parties; one party may benefit if the event occurs, while the other party's obligation may not arise at all.

For example, insurance contracts are typically seen as aleatory because the insurer's duty to pay a claim is contingent upon the occurrence of a loss, an event that is not guaranteed to happen. This uncertainty is a fundamental aspect of aleatory contracts, defining their nature.

In contrast, unilateral contracts involve one party making a promise that becomes binding only when the other party performs a specified act. Onerous contracts require both parties to perform obligations that are mutually beneficial and typically equitable in nature, without any dependency on uncertain future events. Gratuitous contracts, meanwhile, involve one party providing a benefit to another without expecting a return or consideration, lacking the element of uncertainty that defines aleatory contracts.