Final answer:
Binchois and Du Fay made 'art songs' out of plainsong by incorporating additional voices and harmonies around the chant melody.
Step-by-step explanation:
Binchois and Du Fay were composers from the Medieval and Renaissance periods who incorporated plainsong melodies into their polyphonic works, also known as art songs.
They achieved this by taking a chant melody and writing additional voices and harmonies around it, creating a complex and elaborate musical texture.
For example, they might take a plainchant as a starting point and then add multiple voices singing different melodies that intertwine and harmonize with each other.