Final answer:
In an opera, types of complete compositions that can exist and be performed on their own include overtures, intermezzos, entr'actes, arias, ballets, and choruses.
Step-by-step explanation:
Within an opera, there are a variety of complete compositions that can exist and be performed independently on an orchestral program. These include the overture, which is an instrumental introduction to the entire opera, often setting the mood for what is to come; heavy instrumental passages like intermezzos and entr'actes, which can serve as bridges between acts or scenes; and arias, which are solo vocal pieces that showcase a character's emotions and vocal prowess. Another form is the ballet, a composed dance section within the opera, which can be extracted and performed on its own. Lastly, operatic choruses, with their strong vocal harmonies and thematic material, are compositions that also stand alone effectively in concert settings.