Which term would best fit a piece intended to be played in a free and expressive manner?

The term that best fits a piece intended to be played in a free and expressive manner is "rubato." This term, derived from the Italian word for "stolen," indicates a flexibility in the tempo of a piece, allowing the performer to stretch or rush certain notes or phrases without strictly adhering to the written tempo. This technique enhances expressiveness and emotional depth, making it ideal for performances that aim to convey feelings or convey a more personal interpretation of the music.

In contrast, the other terms do not convey the same sense of tempo flexibility intended by "rubato." For instance, "vivace" indicates a lively and brisk tempo, "spiritoso" refers to a spirited, animated style, and "leggiero" means light and delicate. While these terms describe specific performance styles, they do not suggest the interpretative freedom that "rubato" allows, therefore making "rubato" the most appropriate choice for a piece to be played in a free and expressive manner.