Which term indicates a state of peace or tranquility in music?

The term that indicates a state of peace or tranquility in music is "tranquillo." This term is used to convey a serene, calm, and peaceful mood within a musical piece. When musicians see "tranquillo" in the score, they understand that they should perform the music in a relaxed and gentle manner, creating an atmosphere of calmness.

The other terms do not carry the same connotation. "Ostinato" refers to a repeating musical phrase or pattern, often creating a sense of movement rather than tranquility. "Con moto" indicates a lively or spirited tempo, suggesting energy rather than peace. "Doloroso" conveys a sense of sadness or sorrow, which is distinctly different from a peaceful state. Understanding the nuances of these terms helps musicians interpret and express the intended character of a piece accurately.