Which term indicates a soft sound in music?

The term that indicates a soft sound in music is "piano." In musical notation, "piano" is used to instruct musicians to play softly. It is one of the fundamental terms used to denote dynamics, which describe the volume at which music should be played. The term itself translates to "soft" in Italian, which is the basis of its use in musical contexts.

While "pianissimo" also indicates a soft sound, it refers to playing very softly, even softer than "piano." Thus, "piano" is the appropriate basic term for a soft sound, while "pianissimo" is a more specific dynamic marking—making "piano" the correct answer regarding the general indication of softness.

The other terms listed do not refer to softness. "Primo" typically designates the first part or voice in a piece of music, while "prestissimo" indicates a very fast tempo. Understanding these terms within the context of dynamics and musical terminology can greatly enhance one’s comprehension of musical instructions and expressions.