Which of the following terms indicates a slower tempo than what is considered standard?

Meno mosso is the term that indicates a slower tempo than what is considered standard. In musical terminology, "meno" translates to "less," and "mosso" means "moved" or "animated." Therefore, "meno mosso" specifically directs the performer to play at a slower pace than the previous section or than the typical tempo. This contrasts with terms that might denote more moderate or louder dynamics but do not address tempo in the same way.

In the context of the other options, "meno" on its own does imply "less" but must be paired with another term to indicate what is being reduced. "Mezzo piano" refers to a dynamic level meaning "moderately soft," which deals with volume rather than tempo. "Maestoso" means "majestic" and generally suggests a stately, sometimes slower tempo, but it does not specify a reduction from a standard tempo. Thus, "meno mosso" stands out as the clear choice for indicating a deliberate slowing down compared to the standard tempo.