What signifies a key change in a piece of music?

The correct answer is modulation, which refers to the process of changing from one key to another within a piece of music. This shift can create a variety of effects, such as enhancing emotional intensity or providing contrast within a composition. Modulation can occur in different ways, such as through direct modulation, where the change is abrupt, or through pivot chords, which act as a bridge between the two keys.

Transition, while related to the movement between sections of a piece, does not specifically imply a change in key. Refrain usually refers to a repeated section that contains the main theme, while cadence pertains to the musical punctuation at the end of phrases rather than any change in key. Thus, modulation accurately describes the process of key change, making it the correct choice.