The Basic Law of the Federal Republic of Germany, the federal constitution, stipulates that the structure of each Federal State's government must "conform to the principles of republican, democratic, and social government, based on the rule of law" (Article 28). Most of the states are governed by a cabinet led by a Ministerpräsident (Minister-President), together with a unicameral legislative body known as the Landtag (State Diet). The states are parliamentary republics and the relationship between their legislative and executive branches mirrors that of the federal system: the legislatures are popularly elected for four or five years (depending on the state), and the Minister-President is then chosen by a majority vote among the Landtag's members. The Minister-President appoints a cabinet to run the state's agencies and to carry out the executive duties of the state's government.

Which article stipulates that the structure of each Federal State's government must "conform to the principles of republican, democratic, and social government, based on the rule of law"?