Which of the following implies that an act must happen for a contract to be valid?

A Conditional Contract is based on the occurrence of a specific event or condition for it to become effective. This means that the obligations of one or both parties are contingent upon a specific future event happening. For instance, a contract that states payment will only be made if a house passes inspection is valid only if that condition is met. This aspect of a conditional contract makes it inherently different from other types of contracts, which may not require such conditional events for validity.

In contrast, a unilateral contract involves one party making a promise in exchange for a specific act by another party, but the contract does not hinge on the occurrence of a defined condition. An aleatory contract is determined by the occurrence of an uncertain event, but it does not inherently require an act for the contract to exist. A mutual contract refers to agreements where both parties are bound by mutual promises, which does not necessitate the occurrence of a condition for validity.