Which section of a sonata involves the development of themes through key changes?

The correct answer is the section known as the Development. In a sonata form, the Development is characterized by its exploration and transformation of themes presented earlier in the Exposition. This section typically features key changes and modulations that create tension and contrast, often taking the original motifs and varying them in new ways. Composers use the Development to delve into different harmonic and thematic possibilities, which can lead to dramatic contrasts and increased emotional intensity.

The other sections play distinct roles: the Exposition introduces the main themes and establishes the primary key; the Recapitulation restates the Exposition but resolves the harmonic tension by returning to the original key; and the Introduction, if present, serves to set the stage for the work but does not contain the thematic development typical of the Development section.