What musical form consists of a theme followed by variations that change aspects such as melody, key, and rhythm?

The correct answer identifies a musical form characterized by presenting a theme and then altering it through various techniques across successive variations. In the theme and variations form, the initial statement of the theme establishes a musical idea, which is then transformed in a number of ways. These transformations can include changes in melody, harmonic structure, rhythm, dynamics, or texture, allowing the composer to explore different musical expressions while maintaining a link to the original theme.

In this form, each variation provides an opportunity for creativity and innovation, showcasing the composer's ability to maintain listener interest through diverse reinterpretations of the theme. This format is widely utilized in classical music and can be found in compositions by many prominent composers throughout history.

Rondo form involves a recurring theme interspersed with contrasting sections, while sonata-allegro form is structured around exposition, development, and recapitulation, focusing on thematic development rather than variation. Binary form is structured in two contrasting sections without the element of theme variation. Thus, theme and variations is distinctive for its focus on exploring a single musical idea through numerous alterations.