Absolute music refers to music that is not explicitly about anything; it is purely instrumental and does not attempt to convey a narrative or represent external ideas. Among the options you provided:
- An opera: This is not an example of absolute music, as it combines music with a story and often includes lyrics.
- A film score: This is also not absolute music since it is specifically designed to accompany and enhance the narrative of a film.
- A tone poem: While a tone poem can be descriptive and convey a particular theme or idea, it often paints a picture or tells a story, so it's not strictly absolute music.
- A concerto: This is the closest to absolute music, as it is typically an instrumental work that showcases a soloist with an orchestra without an explicit narrative.
Thus, the best example of absolute music among your choices is a concerto.