Which of the following terms describes a musical expression that is soft and sweet?

The term that effectively describes a musical expression that is soft and sweet is "fortepiano." In music, "fortepiano" indicates a dynamic where a note or passage should be played loudly at first (forte), then immediately followed by a decrease in volume (piano). This dynamic creates an expressive contrast that can evoke a gentle or sweet character, especially when used appropriately in a musical context.

In contrast, "fortissimo" refers to a very loud dynamic, which does not align with the idea of softness. "Larghetto" denotes a slow tempo, which might convey a sense of calm but does not specifically relate to the softness or sweetness of sound. Lastly, "grave" indicates a very slow tempo and a serious or somber character, which also does not convey the notion of being soft and sweet. Thus, "fortepiano" stands out as the most fitting term for the description provided.