What best describes the structure of AA'A in music?

The structure labeled as AA'A indicates a form in which there are two sections that are similar, with the second section (the A') being a variation or slight modification of the first (A). This characteristic of having a repeated section that is not identical but is related to the initial section is what aligns with rounded binary form.

In rounded binary form, the A section is often repeated with some alterations in A', followed by a contrasting B section, and then a return to the A section in its original or modified state. The presence of the A' signifies that there is development or variation from the first A, which is a key feature of this form.

In contrast, binary form simply consists of two distinct sections (A and B) without this mutual relationship or variation element, while strophic form typically involves the repetition of the same music for each verse or stanza of lyrics, focusing more on lyrical content than structural variation. Ternary form, on the other hand, includes three sections, typically with an A-B-A structure, which further differs from the outlined AA' form.