What term refers to a human being who is alive, including an unborn child at every stage of gestation?

The term that refers to a human being who is alive, including an unborn child at every stage of gestation, is "individual." In legal contexts, the term "individual" is often used to denote a person as a distinct being with rights and legal capacities. This definition encompasses not only those who have been born but also unborn children, acknowledging their existence as individuals under the law.

In contrast, the other terms provided have different meanings in legal discourse. "Citizen" typically refers to a member of a state or nation with specific rights and responsibilities, which usually applies to those who are born and recognized under the law. "Entity" is a broader term that can apply to organizations, corporations, or any legally recognized collectives, lacking the specific reference to human beings. "Subject" can refer to an individual or entity that is under the jurisdiction of laws but does not convey the same level of personhood or legal recognition associated with human beings as "individual" does. Thus, "individual" is the most accurate choice in capturing both the living status of a human and inclusive coverage of unborn children in a legal context.