Which word best describes a musical element that is calm and tranquil?

The term "tranquillo" specifically denotes a sense of calmness and tranquility in music. It is often used as an instruction for musicians to perform a piece in a serene, gentle manner, evoking peaceful emotions. This word directly conveys the desired quality of stillness and relaxation in a musical context, making it the most fitting choice for describing a calm and tranquil musical element.

In contrast, "doloroso" implies sorrowful or mournful expressions, which, while potentially reflective, do not convey the sense of tranquility sought in this question. "Con moto" translates to 'with motion' or 'with movement,' suggesting liveliness and energy rather than calmness. Lastly, "marcato" suggests an emphasis or accentuation, indicating a more forceful or pronounced approach rather than a serene and tranquil mood. Therefore, "tranquillo" is the most appropriate term to capture the essence of calmness in music.