Which eligible nation has not joined the European Union since its creation?

Switzerland is the correct answer because it has consistently opted not to join the European Union despite its close ties with the organization. Since the creation of the EU, Switzerland has maintained a unique position in Europe, preferring to remain outside the Union while still participating in various agreements that allow for cooperation, such as the Schengen Agreement and the European Economic Area (EEA).

In contrast, Poland, Slovenia, and Greece have all joined the European Union after its establishment. Poland and Slovenia became members in 2004, during the enlargement that welcomed several Central and Eastern European countries, while Greece was one of the earliest members, joining in 1981. Thus, Switzerland stands out as the only nation listed that has never been a member of the EU since its inception.