Which of the following terms refers to a gradual increase in tempo?

The term that refers to a gradual increase in tempo is "accelerando." This musical directive indicates that the performer should gradually speed up their playing or singing, creating a sense of urgency and drive in the music. This technique is commonly used to build excitement or tension within a piece, especially leading up to a climactic moment.

In contrast, the other terms have different meanings. "Ritardando" indicates a gradual decrease in tempo, which serves to slow down the music, often for dramatic effect. "Presto" denotes a fast tempo but does not imply any gradual change; it is a specific tempo marking. "Moderato" refers to a moderate tempo, again without suggesting any gradual change. Understanding these terms helps musicians interpret and perform the music as intended by the composer.