Which term refers to a legal obligation that requires a party to take a specific action or can result in liability?

The correct term that refers to a legal obligation requiring a party to take a specific action or face potential liability is "duty." In legal contexts, a duty signifies a responsibility that one party owes to another, often grounded in ethical, legal, or contractual standards. Fulfilling this duty is essential, and if a party fails to meet this obligation, they may be held liable for any resulting harm or damages.

In contrast, the other terms do not encapsulate the notion of a legal obligation in the same way. An affidavit is a written statement confirmed by oath or affirmation, used as evidence in court. A contract is an agreement between parties that may create obligations, but it is the duty arising from that contract that specifically refers to the obligations required. A brief is a document that outlines legal arguments or points of law, typically submitted to a court, and does not embody the concept of a legal duty. Thus, "duty" is the term that most accurately describes a legal obligation leading to liability.