Which of the following European languages is classified as a "language isolate?"

Basque is classified as a language isolate because it does not have any known linguistic relatives. It is spoken in the region of Basque Country, which spans parts of northeastern Spain and southwestern France. Despite its presence in Europe, Basque is not related to any other language in the world. While most European languages belong to the Indo-European language family, Basque stands apart as a unique language with a distinct grammar, vocabulary, and syntax. Its isolation has led to significant interest and study among linguists, as its origins and development remain a puzzle yet to be fully unraveled.