What is the name of a three-part form (ABA) where section B creates contrast in key or material?

The three-part form known as ABA is referred to as Ternary form. In this structure, the first section (A) presents a theme or musical idea, which is then followed by a contrasting section (B) that often introduces new material or changes in key. The return of the first section (A) at the end brings back the initial theme, creating a sense of balance and symmetry within the composition.

The contrasting middle section is pivotal in providing variety and depth, allowing composers to explore different harmonies, melodies, and textures that enrich the overall piece. This ability to juxtapose different musical ideas is what distinguishes Ternary form and highlights its effectiveness in creating a cohesive musical experience.

The other choices do not fit the description of this specific three-part structure. Binary form consists of two distinct sections (AB) without a return to the initial theme. Quaternary form implies a structure with four distinct sections, which does not apply here. Round form typically refers to a type of canon or contrapuntal piece rather than a straightforward ABA structure. Thus, Ternary form is accurately identified as the correct answer for this question.