Which term describes a musical passage that should be played "slow and solemn"?

The term that describes a musical passage to be played "slow and solemn" is indeed grave. In music, "grave" indicates a very slow tempo, often associated with a serious or somber mood. This term sets the emotional tone and character of the piece, suggesting a dignified and reflective quality.

Understanding the nuances of tempo terms can greatly enhance interpretation. Grave is typically slower than other tempos like "adagio" or "largo" and emphasizes a heavy, serious nature in the music being played. This choice aligns perfectly with the requirements for a slow and solemn interpretation.

The other options do not match the description of being both slow and solemn. "Giocoso" refers to a lively, playful style. "Energetic" suggests a fast and vigorous approach, while "expressive" focuses on conveying deep feelings but does not specify tempo or solemnity. Thus, grave is the most appropriate term for this context.