Which of the following describes a "slowing down" directive in music?

The term "rallentando" refers to a gradual slowing down of the tempo in music. It instructs musicians to decrease the speed at which they are playing, creating a feeling of relaxation or tension release in the music. This directive is often used to enhance emotional expression or to prepare for a transition in a piece.

In contrast, the other terms refer to different concepts in musical dynamics or tempo. "Mezzo piano" indicates a moderate soft volume level, “a tempo” refers to returning to the original tempo after a deviation, and “prestissimo” signifies playing as fast as possible. None of these terms imply a slowing down of the tempo, making "rallentando" the correct choice for describing a slowing down directive in music.