What term describes a polyphonic vocal composition featuring multiple melodic lines?

The term that best describes a polyphonic vocal composition featuring multiple melodic lines is the Medieval Motet. In the context of music history, motets emerged during the Medieval period and were characterized by the interweaving of independent melodies sung at the same time. This form allowed for rich harmonic textures and intricate musical structures, as composers crafted various parts that would complement each other while maintaining their own distinct melodic identities.

The Medieval motet often incorporated sacred texts and was used in liturgical settings, marking it as an essential precursor to later developments in polyphony. In contrast, the other options represent different music forms or genres. Renaissance choral works indeed include polyphonic characteristics, but they evolved from the earlier motet style. Baroque opera is a theatrical genre involving vocal and instrumental components, while Romantic symphonies focus on orchestral compositions rather than vocal polyphony. Understanding the unique context and characteristics of these music types helps clarify why the Medieval motet is the correct term for this question.