Final answer:
The solo instrument in 'Spring' from 'The Four Seasons' by Vivaldi is the violin, conveying the joyful renewal of the season.
Step-by-step explanation:
The solo instrument featured in Spring from The Four Seasons by Antonio Vivaldi is the violin. Composed in 1723, this piece is part of a group of four violin concerti, each of which gives a musical expression to a season of the year. Vivaldi's Spring concerto is particularly noted for its bright, joyful melodies that evoke the sense of nature's renewal associated with the season.