What does modulation refer to in music?

Modulation in music specifically refers to the act of changing from one key to another within a piece. This process introduces new harmonic landscapes and emotional textures, providing contrast and development within the music. By moving to a different key, composers and performers can enhance the narrative and complexity of the piece, often leading to different sections that may feel dramatically distinct from the original key.

While the other options describe various aspects of musical performance and structure, they do not encapsulate the specific process of modulation. For instance, changing tempo refers to performing at a different speed, and creating variations in dynamics involves altering the volume levels of notes and phrases, neither of which changes the key of the piece. Repeating a phrase multiple times contributes to the structure of the piece but does not involve a change of key. Thus, modulation's definition is centered around the transition between keys, making the choice of changing from one key to another the correct answer.