Which of the following terms refers to the legal obligation to act in a certain way?

The term that refers to the legal obligation to act in a certain way is "Duty." In legal contexts, a duty can arise from statutes, contracts, or common law, mandating an individual to act or refrain from acting in certain ways. For example, a duty of care requires individuals to act with a certain standard of conduct toward others to avoid causing harm. This concept is fundamental in various areas of law, including tort and contract law, where the failure to fulfill a duty can lead to liability.

Authority, while related to the ability to act on behalf of another, does not denote an obligation. Agency refers to the relationship where one party (the agent) acts on behalf of another (the principal) and is closely tied to authority but again does not specifically denote an obligation. Privilege relates to special rights or advantages granted to an individual or group, which does not convey an obligation to act in a certain manner.