Music appreciation. Which section of a classical concerto could be improvised?

A. The modulation B. The prelude C. The cadenza D. The development

I would say the correct answer is C. the cadenza.

Cadenza is defined as " an improvised or written-out ornamental passage played or sung by a soloist or soloists,' so you can see that the important part of the cadenza is that it is improvised. It is used to show the skill of the performer.