Which term refers to a sad or mournful musical expression?

The term "mesto" refers specifically to a sad or mournful musical expression. It is often used in musical directions to convey a sense of sorrow or melancholy in the performance. Composers use this term to instruct musicians to interpret the music with a somber tone, contributing to the overall emotional impact of a piece.

In contrast, morendo conveys a sense of dying away or fading, which indicates a gradual decrease in volume and intensity rather than focusing specifically on sadness. Pesante, on the other hand, means heavy or ponderous, suggesting a weighty or serious character but not necessarily mournful. Mezzo forte indicates a medium loud dynamic level, referring to volume rather than emotional content. Thus, while other terms describe different aspects of musical expression, "mesto" distinctly captures the quality of sadness, making it the correct answer in this context.