Which of the following operas by Wagner is not part of the tetralogy "Der Rings des Nibelungen" (The Ring of the Nibelung)?

Explanation

Wilhelm Richard Wagner (1813-1883) was a German composer, best known for his operas. Wagner was born in Leipzig and died in Venice. He wrote "Der fliegende Holländer" (1843), "Tristan und Isolde" (1865), "Parsifal" (1822) and the four operas in the "Der Ring des Nibelungen" (1848-1874): "Das Rheingold", "Die Walküre", "Siegfried" and "Götterdämmerung". The tetralogy is based on characters and themes from Norse mythology.