If Haydn did not use a theme and variations for the second movement of his string quartet in C major, which alternative form(s) might he have chosen?

The second movement of Haydn's string quartet in C major is most famously known for its theme and variations structure. If he had chosen not to use this form, the sonata form or the ABA form would have been viable alternatives.

Sonata form is highly significant in classical music, combining elements of exposition, development, and recapitulation, allowing for thematic exploration and contrast. This form would provide a rich structure that aligns well with classical composition practices, making it a fitting option for a second movement.

On the other hand, the ABA form, which consists of a statement of the initial theme, a contrasting section, and a return to the theme, offers a simple yet effective way to create an emotional arc within the movement. This would allow Haydn to maintain thematic cohesion while exploring different musical ideas.

While the other forms listed, such as through-composed or rondo, would lead to different compositional outcomes, they do not represent the traditional forms commonly associated with the second movements of string quartets in the classical era as effectively as sonata or ABA forms do. Thus, choosing sonata or ABA as alternatives aligns closely with Haydn's stylistic tendencies and common practices of the time.