As the picture suggests, this is about rivers in Germany. "German Corner" ("Deutsches Eck" in German) is the point where two great rivers meet, the Rhine and the Moselle. All four answer options are German cities that are located on a river:

Cologne is on the lower Rhine.

Wiesbaden is on the upper Rhine.

Trier is on the Moselle.

Koblenz stands on both rivers and is the location of "German Corner".

Koblenz was established as a Roman military post in about 8 BC. Its name originates from the Latin (ad) cōnfluentēs, meaning "(at the) confluence". "German Corner" is the promontory at this confluence (in the centre of the picture) and became known for a monumental equestrian statue of William I, first German Emperor, erected in 1897 in appreciation for his role in the unification of Germany. The monument was destroyed in World War II and only the plinth was preserved as a memorial.

Following German reunification, a replica of the statue was erected on the pedestal in 1993 and has become a Koblenz landmark. Today, a national flag and the flags of the 16 “Länder” (the German states) fly at German Corner as a reminder of German unity. In addition, the flag of the United States of America is displayed as a sign of support for the victims of the September 11 attacks. There are also three parts of the Berlin Wall, now dedicated to the "victims of the division".

More Info: en.wikipedia.org