Final answer:
The cellos and double basses in the background of this passage play the function of harmony.
Step-by-step explanation:
The cellos and double basses in the background of this passage play the function of harmony. Harmony refers to the combination of different musical notes played together to create chords and support the melody.
In this case, the cellos and double basses provide a rich, deep sound that supports the overall harmony of the piece. They play sustained notes and provide a lush, full texture to the music.