Final answer:
A jus soli citizen is someone born on the country's territory according to the Fourteenth Amendment of the U.S. Constitution. This principle of 'law of the soil' embodies the idea of birthright citizenship, making any child born in the U.S. a citizen regardless of parental citizenship status.
Step-by-step explanation:
The term jus soli refers to the right of anyone born in the territory of a state to nationality or citizenship. As it pertains to the United States, the Fourteenth Amendment to the Constitution embodies this principle by stating that anyone born on U.S. soil is a citizen of the United States, regardless of the citizenship of their parents. This amendment, combined with the legal principle, establishes the basic criteria for soli citizenship status within the country.
Therefore, from the options provided, a. born on the country's territory is the correct one that describes a jus soli citizen in the scope of U.S. law. Choices b, c, and d rather refer to other ways of obtaining citizenship, such as through adoption, naturalization, or jus sanguinis ('right of blood' or citizenship by descent).
In historical contexts, such as with Roman citizenship, to gain civitas at birth, it was necessary to be the child of two citizens, which reflects the jus sanguinis principle. By contrast, the modern U.S. principle of jus soli makes no such requirement, instead granting citizenship to all those born within its territory unconditionally, with specific exceptions such as children of foreign diplomats.