What is the primary function of a key signature?

The primary function of a key signature is to clarify the scale and its accidentals, making option C the most accurate choice. A key signature is a collection of sharp or flat symbols placed at the beginning of a staff in a piece of music, indicating which notes should be consistently altered throughout the composition. This allows musicians to understand the tonal center of the piece and the specific scale being used, which directly influences the melody and harmony.

In contrast, changing the tempo relates to the speed at which the music is played and is not determined by the key signature. The volume level deals with dynamics, which specifies how loudly or softly music should be played, also independent of the key signature. Providing chord notations, while important in music, is typically included elsewhere in the music notation and does not stem from the key signature.