During what period did Guillaume de Machaut compose his most notable works?

Guillaume de Machaut composed his most notable works during the 14th century, a time significant for the development of polyphonic music in the medieval period. He is one of the most prominent figures of that century and is often credited with being a pioneer of the Ars Nova style, which introduced new approaches to rhythm and notation. Machaut's contributions include both sacred and secular music, and he is well-known for writing the first complete setting of the Mass Ordinary by a single composer, known as the "Messe de Nostre Dame." His works showcase intricate melodies and innovative structures that influenced future generations of composers. While other periods mentioned in the choices contributed to music history, they do not align with Machaut's principal compositions, making the 14th century the correct context for his notable achievements.