Which marking indicates that a section of music should be played at a 'moderate speed'?

The marking that indicates a section of music should be played at a 'moderate speed' is 'Moderato.' This term is part of the Italian musical lexicon used to describe tempo, and it specifically denotes a moderate pace that is neither too fast nor too slow. This gives performers clear instructions on the desired speed for that musical passage, which is fundamental for maintaining the intended character and flow of the piece.

The other terms, while they have specific meanings in music, do not refer to a moderate tempo. For example, 'Maestoso' typically suggests a majestic and dignified tempo that is often slower and more grandeur-laden than moderato. 'Meno' translates to 'less,' but does not provide a direct tempo indication. 'Misterioso' indicates a mysterious quality or mood rather than a speed, often suggesting that the music should evoke a sense of intrigue or enigma. Each of these terms conveys different musical instructions, but it is 'Moderato' that clearly defines a moderate speed.