Explanation:

- A contingent contract is a contract that depends on the occurrence or non-occurrence of a specific event in the future.

- Option 1, main event: This refers to the primary objective or goal of the contract itself, not a condition for the contract's performance.

- Option 2, collateral event: This is an event that triggers the performance of the contract. In a contingent contract, it is the collateral event that impacts whether the contract will be executed.

- Option 3, both (a) and (b): Suggests performance depends on both main and collateral events, which is typically not how these contracts function.

- Option 4, either (a) or (b): Indicates dependency on either event, which misstates the nature of contingent contracts.

Correct Answer: Option 2, collateral event