Explanation:

- Forbidden by Law (Option 1): If the consideration or object of an agreement is illegal, it is clearly not valid.

- Defeat the Provisions of any Law (Option 2): If the agreement negates or undermines legal provisions, it becomes unlawful.

- Either (a) or (b) (Option 3): If either the consideration or object of an agreement is forbidden by law or it defeats the provisions of any law, the agreement is unlawful.

- Neither (a) nor (b) (Option 4): This option implies that both conditions do not cause unlawfulness, which is incorrect.