The correct answer is: religion.
We know this because the ancient Greek religion is no longer practiced, neither in Greece, nor in Rome/Italy (it was mixed with the Roman Religion and practiced there for a long time), nor in the rest of Europe or the world.
The other fields are full of words and concepts of Greek origin, such as : Aristotle's work (philosophy),the word "heliocenticism"( the English language and science ) and medicine- "artery" is a Greek word.