Question

An instruments-only piece played at the opening of an opera is known as a/an

Answer 1

The correct answer is D. Overture

An overture is basically an introduction. It consists of only instruments and usually presents the musical themes that will be seen later during an opera. Overtures always come in the beginning and don't include any singing or similar things that might distract from the music since the voices will come at a later time.

Answer 2

D. Overture 
that's the answer!