What term refers to a musical passage that is loud and then suddenly soft?

The term that accurately describes a musical passage that is loud and then suddenly soft is "fortepiano." This term specifically indicates a dynamic contrast in music, where the sound starts off loud (forte) and then immediately drops to soft (piano). This abrupt change in dynamics adds expressiveness and nuance to the music.

Other terms in the choices refer to different aspects of dynamics. "Forte" means loud, "fortissimo" signifies very loud, and "piano" means soft. While "forte" and "piano" refer to static states of volume, they do not capture the essential element of the sudden dynamic shift that "fortepiano" does. Fortepiano is used in musical notation to convey this specific effect, making it the correct answer in this context.