Vivaldi's "Winter" feels complete and unified for which reason?

The sense of completeness and unity in Vivaldi's "Winter" primarily comes from the fact that it begins and ends in the key of F minor. This tonal consistency helps to create a cohesive musical experience as the work progresses. Starting and concluding in the same key establishes a framework that gives listeners a feeling of return and resolution, a hallmark of effective musical composition. This unity in tonality supports the emotional and atmospheric qualities that Vivaldi aims to convey, reflecting the stark and often harsh characteristics associated with winter.

The other choices do not contribute as directly to the overall unity of the piece. While a complex chord structure can enrich the music, it does not inherently provide a sense of completion in the way that a consistent key does. The presence of solo instruments adds to the texture and distinctiveness of the performance but does not imply the same level of overall unity. Lastly, though Vivaldi's "Winter" may be inspired by a poem, this literary aspect doesn't specifically determine the structural cohesiveness of the music itself in the way that its tonal framework does.