Explanation:

- Option 1: Substantive Law: This pertains to laws that define rights and duties, such as statutes or case law, and provide the framework within which individuals live and interact.

- Option 2: Rule of Equity: These are rules developed by courts to ensure fairness, often filling gaps left by the law, and include principles like estoppel to prevent unfair advantage.

- ? Option 3: Rule of Evidence: This category includes doctrines that govern the admissibility of evidence in court. Estoppel functions to prevent parties from asserting contradictory positions and is used within the context of evidence.

- Option 4: Law of Pleadings: This concerns the formal procedures and documents needed to present a case in court, such as complaints and defenses.