Which term best describes a musical piece with a single melody and harmony?

The term that best describes a musical piece with a single melody and harmony is "homophony." In homophony, there is one clear melodic line that is supported by accompanying chords, which creates a harmonious texture. This is often the typical structure of many popular songs, where a singer's melody is complemented by instrumental chords.

In contrast, other terms denote different textures in music. A "sonata" refers to a specific form that typically consists of several contrasting movements, not just a single melody with harmony. "Polyphony" describes a texture where multiple independent melodies are played or sung simultaneously, creating a complex interweaving of voices, which is distinct from the single melody and harmony of homophony. "Monophony" refers to music that consists of a single melodic line without harmonic support or accompaniment, which again differs from the concept of having both a melody and harmony present. Thus, homophony is the most accurate term for a piece featuring one melody supported by harmony.