Which of the following describes a gradual change in tempo?

The term that describes a gradual change in tempo is "poco a poco." This phrase is Italian for "little by little" and is commonly used in musical notation to indicate that the performer should gradually increase or decrease the tempo over a specific section of the music. This concept allows for an organic and expressive pacing in a piece, emphasizing the emotional arc of the music.

In contrast, "accelerando" refers specifically to a gradual increase in tempo, while "ritardando" denotes a gradual decrease in tempo. "Adagio," on the other hand, indicates a slow tempo rather than a change in tempo. Each of these terms serves a distinct purpose in musical notation, with "poco a poco" being the correct choice for describing a gradual change without specifying the direction of that change.