Which of the following terms refers to a sustained manner of playing?

The term "sostenuto" indeed refers to a sustained manner of playing. In music, sostenuto indicates that notes should be held longer than their written value, creating a sense of elongation and continuity. This technique is often used to evoke a more lyrical and expressive quality, allowing melodies to sing and resonate through a sustained sound.

In contrast, the other terms do not imply a sustained manner of playing. "Sforzando" is a directive to play a note with a sudden, strong emphasis, which contrasts with the idea of sustaining. "Spiritoso" suggests a lively and spirited manner of playing, focusing more on energy and vivacity rather than sustenance of sound. "Simile" instructs the performer to continue in the same style or manner as previously indicated, but it does not specifically relate to sustaining notes. Thus, "sostenuto" is the correct choice as it directly addresses the concept of sustaining in performance.