Which of the following indicates a very soft dynamic in music?

The term that signifies a very soft dynamic in music is "pianissimo." This marking instructs musicians to perform with great softness, typically at a level softer than "piano." It is used to create a delicate and intimate atmosphere, often conveying subtle emotions or nuances within a piece. The indication of "pianissimo" is often denoted by "pp" in musical notation.

The other dynamics mentioned represent varying degrees of softness or loudness but are not as soft as pianissimo. "Mezzo piano" indicates a moderately soft dynamic, while "piano" itself suggests a soft dynamic but is louder than "pianissimo." "Mezzo forte" indicates a moderate loudness and is not in the realm of softness at all. Understanding these terms helps in interpreting the intensity and emotional context of a musical piece.