A contract where both parties gain a benefit is known as?

A contract where both parties gain a benefit is referred to as onerous. In an onerous contract, there is a mutual exchange of benefits between the parties involved, meaning each party provides something of value to the other. This contrasts with other types of contracts that may not involve such a reciprocal benefit.

For instance, a gratuitous contract involves a benefit to one party without any corresponding obligation from the other, making it a one-sided benefit. On the other hand, a unilateral contract is when only one party makes a promise or performs a duty without an exchange from the other party, which also does not provide a mutual benefit.

An aleatory contract, meanwhile, is dependent on an uncertain event, where one party may gain a benefit while the other may not actually gain anything at all, depending on the outcome of that event. In contrast, an onerous contract ensures that both parties actively contribute to and receive something of value from the agreement, which is why it is the correct answer in this context.