Which term would best describe an expressive, flowing performance?

The term that best describes an expressive, flowing performance is "Adagio." This term indicates a slow tempo, which allows for more expressive playing and the opportunity for the performer to convey emotions deeply. In an adagio passage, musicians typically have the flexibility to engage with the nuances of the music, presenting it in a lyrical manner that enhances its expressiveness.

Though there are other terms listed, they have different implications that do not align with the idea of a flowing, expressive performance. "Accelerando" refers to a gradual increase in tempo, which might lend excitement but does not specifically relate to expressiveness or flowing quality. "Agitato" suggests a restless and agitated character, often marked by a more frantic or intense expression, rather than the calm and flowing quality of an adagio. "Allargando" means to gradually broaden the tempo or slow down, which can lead to a more grand or dramatic finish, but it doesn’t convey the ongoing flowing nature that 'Adagio' does. Therefore, 'Adagio' is the most fitting term for an expressive, flowing performance.