In nineteenth-century opera, the orchestra: a. became less important as vocal lines became more important. b. was reduced to a small group as opera houses became smaller. c. became more important than the vocal lines; the voices now accompanied the orchestra. d. increased in importance, sometimes providing special effects and overshadowing the voices.

D. increased in importance, sometimes providing special effects and overshadowing the voices.

Explanation: In nineteenth-century opera, the orchestra increased in importance, sometimes providing special effects and overshadowing the voices. An opera is a form of art in which the musicians and the singers combine music and text. It is usually performed in opera houses accompanied by an orchestra. The Romantic period was dominating the arts and the opera. There was a burst of interest for historic themes, supernatural beings, and nationalism, and these theme were displayed in operas.