The correct option is C. An overture is an instrumental piece that typically introduces an opera. For instance, Beethoven and Mendelssohn used overtures in the Romantic Era before their pieces were performed.
Overture in music was originally the instrumental introduction to a ballet, opera, or oratorio in the 17th century. During the early Romantic era, some composers, including Beethoven and Mendelssohn composed overtures which were independent, self-existing instrumental, programmatic works that presaged genres such as the symphonic poem.