What characterizes a 'duo sonata' in musical composition?

A duo sonata is characterized primarily by the combination of a solo instrument and piano. In this format, one musician plays an instrument—commonly a violin or cello—while the pianist provides harmonic support and accompaniment. This collaboration allows for dialogue between the two parts, showcasing the expressive capabilities of both the solo instrument and the piano.

The duo sonata is a popular form from the Classical and Romantic periods, often consisting of several movements that highlight the interplay between the two instruments. Each part is typically written in a way that allows for virtuosic moments, enabling both players to shine individually while also working together musically.

In contrast, the other options do not capture the essence of a duo sonata. A solo performance lacks the collaborative aspect that defines the duo format. A full orchestra with conductor refers to a much larger ensemble that includes multiple instruments and sections, thereby straying far from the duo concept. Two string instruments only could describe other forms of chamber music but do not necessarily meet the specific definition involving a piano.