The anniversary helps with the date; the name helps with the subject.
2007 minus 50 years gives 1957. That was the year in which the six founding countries signed the Treaties of Rome. The word “Economic” in EEC points towards shared economic cooperation.
The treaties built on earlier steps: the countries had already cooperated in coal and steel. Cooperation now expanded into other economic sectors. The EEC therefore forms part of the history leading to today’s EU.

The other three answers
The other proposals concern military alliances, the consequences of war or borders.
Germany's accession to NATO
The Federal Republic joined NATO in 1955. NATO is a military alliance; the 1957 Treaties of Rome established different European communities.
Germany's obligation to pay reparations
Reparations are payments or other contributions to compensate for war damage. The treaties asked about here established shared European economic and atomic-energy communities.
Establishment of the Oder-Neisse line as the eastern border
Establishing an eastern border is a territorial matter. It is not the founding purpose of the Treaties of Rome asked about here.
Two treaties and two different dates.
The plural “Treaties of Rome” matters: the European Atomic Energy Community, Euratom, was established alongside the EEC. The exam answer names the EEC, but is not a complete list of everything in the treaties.
The treaties were signed in 1957 and entered into force on 1 January 1958. The fiftieth anniversary in 2007 refers to their signature. Do not confuse the signature date with the start of their legal effect.
Treaties of Rome, 1957 · European Union ↗Rome, 1957: EEC and Euratom; the exam answer names the EEC.
Connect Rome, 1957 and the Economic Community. Entry into force follows in 1958.
Three words for this question
- das Jubiläumanniversary
- A commemorative year marking a particular interval since an earlier event.
- die Wirtschaftsgemeinschafteconomic community
- A group of countries cooperating within a shared economic framework.
- in Kraft tretento enter into force
- To begin having legal effect.
