Explanation:

- Void: A contract becomes void if it is no longer enforceable by law. If A goes mad, the contract cannot be performed, as A is incapable of understanding the nature and consequences of the marriage contract. Therefore, the contract to marry becomes void.

- Illegal: A contract is illegal if it involves actions that are unlawful or forbidden by law. Here, the contract is not illegal, as marriage is not against the law.

- Valid: A valid contract is legally binding and enforceable. However, A's mental incapacity means the marriage contract cannot be valid.

- Voidable: A contract is voidable if one party can choose to set it aside. In this case, the contract to marry is not voidable but void due to A's mental incapacity.