If a piece of music is described as 'sad', which term might be used?

The term 'mesto' is indeed the appropriate choice when describing music that conveys a sense of sadness. This Italian term translates to 'sad' or 'sorrowful' and is commonly used in music to indicate a melancholic mood or character. Composers might use this term to guide performers in interpreting the emotional quality of a piece, encouraging them to evoke a feeling of sadness through dynamics, tempo, and phrasing.

In contrast, the other options represent different concepts. 'Misterioso' refers to a mysterious or secretive character, suggesting an enigmatic mood rather than sadness. 'Maestoso' indicates a majestic or dignified manner, often associated with grandeur and nobility, which is quite the opposite of a sad expression. Lastly, 'Massig' (translated from German, meaning 'moderately') describes a tempo or style that is moderate, but it does not convey any specific emotional quality like sadness. Thus, 'mesto' is the term that most directly aligns with the description of a piece of music as 'sad.'