Which of these musical elements is usually not altered for improvisation? A. formB. harmonyC. melodyD. rhythm

I think the answer is B

The answer is B. Harmony.

Explanation:

Improvisation is a skill that musicians have, which serve as compositional resources. Musical improvisation is the art of creating and performing music that has not previously been written and that arises spontaneously through a series of material. improvisation usually respects the harmony of the original song, while the rhythm and melody vary depending on the skill and ingenuity of the musician.