What defines a contract where the performance of obligations depends on an uncertain event?

The choice of "C. Aleatory" is accurate because it specifically refers to contracts where the obligations of one or both parties are contingent on an uncertain event occurring. In an aleatory contract, the outcome is uncertain, and performance may depend on an event that may or may not happen, such as in insurance contracts where payment is made only if a certain event (like a loss, damage, or accident) occurs.

This type of contract is characterized by the idea that the parties involved do not have equal risk; instead, one party typically has a greater risk of loss or gain based on the outcome of the uncertain event. Examples include insurance policies and gambling agreements, where outcomes are not guaranteed.

The other types of contracts mentioned do not reflect these conditional, uncertain obligations. Onerous contracts require both parties to perform obligations that are typically a mutual exchange for consideration, while gratuitous contracts involve one party providing a benefit without receiving anything in return—these do not hinge on uncertain events. Unilateral contracts involve an obligation from only one party, where one party makes a promise in exchange for an act by the other party, but again, this does not inherently involve the uncertainty of an event affecting performance like an aleatory contract does.