The statement that best explains the alliance of nations in Europe after WWII is: "All nations in Europe joined either NATO or the Warsaw Pact."
This reflects the division of Europe during the Cold War into two main opposing military alliances: NATO (led by the United States and including Western European nations) and the Warsaw Pact (led by the Soviet Union and including Eastern European nations). While there were some exceptions with non-aligned nations, the majority of European countries aligned themselves with one of these two blocs.