What defines an "adult" as per the relevant legal terms?

The correct definition of an "adult" within legal terms is typically that of a person who is 18 years or older. This age threshold is established by law in many jurisdictions as the age at which an individual is considered to have the legal capacity to make decisions for themselves and engage in contracts, among other rights and responsibilities.

While other choices may relate to legal status, they do not serve as a comprehensive definition of adulthood. For instance, the ability to legally possess alcoholic beverages can vary from one jurisdiction to another; not all places set the same minimum age for this privilege, and some may even allow certain exceptions. Furthermore, while a designated status by the state, such as guardianship or other legal recognitions, could pertain to an individual's legal responsibilities, they do not specifically indicate that the individual has reached adulthood.

In summary, being 18 years or older is a universally recognized criterion for legal adulthood, encompassing a full range of legal rights and responsibilities, which is why this definition is the most appropriate and widely accepted.