What genre represents the earliest form of medieval polyphonic music?

The correct answer is organum because it represents one of the earliest forms of polyphonic music that emerged during the medieval period. Organum developed as a way to enrich Gregorian chant by adding one or more melodic lines to the existing chant melody. This practice began around the 9th century and marked a significant evolution in the development of Western music, as it laid the groundwork for the more complex polyphonic textures that would follow in later centuries.

In organum, the added melodies, which are typically sung in parallel motion to the chant, create harmonic intervals, resulting in the first instances of polyphony. This contrast with monophony—where only a single melody is sung without accompaniment or harmonies—illustrates the importance of organum in music history. The understanding of how different voices can interact set the stage for the rich tapestry of musical styles that would develop later on.

The other options, while related to music of the medieval period, do not represent this earliest form of polyphony as directly. Motets are later developments that incorporate polyphonic textures, secular music refers to non-religious music that gained prominence after the initial development of polyphony, and monophony solely consists of single-line melodies without harmonization.