Which term refers to a slight slowing down and softening of the music at the end of a phrase?

The correct term for a slight slowing down and softening of the music at the end of a phrase is known as a fermata. A fermata indicates that a note or a rest is to be held longer than its usual duration, allowing for a moment of pause and expression. Typically, this occurs at the end of a musical phrase, creating a sense of completion or finality, which aligns perfectly with the concept of softening the music at that point.

While rallentando refers to a gradual decrease in speed, it does not specifically focus on softening as part of its definition. Ritenuto, too, suggests a holding back or a sudden slight slowing, but it also does not emphasize the softening aspect as deeply as a fermata. Stretto typically refers to a quicker, more intense section, especially found in contrapuntal music, and is not related to slowing down or softening music at the phrase's end. Therefore, fermata is the most suitable term in this context.