Which term refers to medieval polyphony that incorporates a Gregorian chant?

The correct term for medieval polyphony that incorporates Gregorian chant is "Organum." This musical form was developed in the early Middle Ages and often involved adding one or more voices to an existing Gregorian chant melody. Organum exemplifies the early stages of polyphonic music, where the original chant serves as the foundation, and new melodic lines are composed above it. This technique not only enhances the harmonic richness but also preserves the integrity of the plainchant.

The other choices represent different types of vocal music. A motet is a more complex vocal composition that developed later, often using different texts and harmonies that do not primarily rely on Gregorian chant. A madrigal is a secular form of vocal music originating in Italy that emphasizes expressive text setting, rather than the liturgical associations found in organum. Lastly, a fugue, which emerged much later in music history, is a counterpoint technique characterized by the systematic imitation of a principal theme, rather than the direct incorporation of a chant. Understanding these distinctions clarifies why organum is the most fitting term in this context.