What is the name of the musical form for solo bagpipe that features themes and variations?

The correct answer is Pibroch, which is a traditional form of music specifically associated with solo bagpipe performance, particularly the Highland bagpipe. This form is characterized by a theme followed by a series of variations that elaborate on the initial melody. Pibroch typically consists of a series of increasingly complex variations that showcase the virtuosity and technical skill of the player, making it a significant and respected form within Scottish music.

In contrast, a sonata generally involves a structured composition for instruments (often a keyboard and another instrument) and follows the sonata form, which is distinct from the thematic and variation nature of a pibroch. A suite is a collection of dance-inspired movements, which may not focus on themes and variations in the same manner as a pibroch. Rhapsody, on the other hand, tends to be a piece characterized by free form and structure, often reflecting improvisational styles and emotional expression, rather than a strict theme and variations format typical of pibroch.