Explanation:

- A contingent contract is a contract where the performance depends on the occurrence or non-occurrence of an uncertain event.

- The event should not depend on the promisor’s own will, as this would make the contract potentially unenforceable.

- Option 1: Mere will - The event must not be within the mere will of the promisor because it should be an external event beyond the promisor's control.

- Option 2: Act - An event dependent on an act might imply actions by third parties or external factors, which can be validly contingent.

- Option 3: Rules - If an event depends on rules, it might refer to external regulations or laws, which can also form the basis of a legitimate contingent contract.

- Option 4: Principles - Principles could refer to standards or doctrines, which don't typically form the basis of contingency.