Which of the following is a key work by Duccio?

Duccio di Buoninsegna, an early Italian painter from the Sienese school, is best known for his contributions to the development of painting during the Late Middle Ages. One of his most significant works is "The Virgin and Child Enthroned," which showcases his skill in creating depth, emotion, and a sense of narrative through detailed figures and vibrant color. This altarpiece is particularly notable for its intricate depictions of saints, rich ornamentation, and a harmonious composition that reflects the Byzantine influence while also paving the way for the burgeoning Gothic style.

In contrast, the other options are key works by different artists: "St. George and the Dragon" is attributed to the artist Donatello; "The School of Athens" was painted by Raphael and is known for its depiction of classical philosophy; "The Birth of Venus" is famously created by Sandro Botticelli, representing the emerging themes of the Renaissance. Each of these artists has their distinct style and context, which places Duccio’s "The Virgin and Child Enthroned" as a critical piece in understanding the transition of art during his era.