A system in which the national government alone has sovereignty (final governing authority) is called:

The correct answer is C. Unitary.

A unitary system is one in which the national government holds the primary power and sovereignty. This means that any regional or local governments derive their authority from the central government, and the central authority has the final say in governance. In contrast, a federal system divides power between national and regional authorities, and a confederate system unites independent entities under a weaker central authority.