Final answer:
The instrumental introduction to the opera is called an overture, which serves to introduce musical themes and set the mood.
Step-by-step explanation:
The instrumental introduction to the opera is called an overture. Option c is correct. The overture serves as a prelude to the opera, introducing themes and setting the mood before the curtain rises. Unlike the libretto, which is the text or script of the opera, the overture is purely musical. The Mannheim rocket is a dramatic ascending musical figure and not an introduction, and the finale is typically the concluding part of an opera. Therefore, the overture is the correct term for the instrumental introduction to an opera.