German unity was achieved through accession in 1990.

Do not confuse accession with conquest.

On 23 August 1990 the freely elected People’s Chamber decided on accession effective on 3 October. The Unification Treaty was signed on 31 August. Decision, treaty and effective date are different stages of the same process.

The correct answer collectively refers to Brandenburg, Mecklenburg-Western Pomerania, Saxony, Saxony-Anhalt and Thuringia as the five eastern states. Unity did not turn them into five new independent countries. They became constituent states of the Federal Republic.

The other three answers

Two answers claim occupation; one reverses the direction.

The short test answer summarises a complex process.

Article 23 of the Basic Law as it stood then provided the basis for accession. Historical references to that article concern its wording at the time. The Unification Treaty addresses Berlin separately from the five states.

Legal accession does not mean that every political issue was uncontested or that its later effects were identical for everyone. The question tests the constitutional route, not an overall assessment of reunification.