Final answer:
The triad became the cornerstone of Western music with the rise of tonality, solidifying during the Baroque period following the polyphonic developments of b. the late Renaissance.
Step-by-step explanation:
The triad has been the fundamental building block of Western music essentially since the end of the Renaissance period and the rise of tonality. This shift in musical structure became more concrete during the Baroque period, which succeeded the Renaissance around the year 1600.
During the Renaissance, music transitioned from modal to tonal qualities, with the increased use of root movements in fifths. These changes were part of the broader context of a society experiencing significant changes like humanistic thought, recovery of ancient Greek and Roman heritage, and the Protestant Reformation. This led to the development of a common, unified musical language, which then gave way to the Baroque period where the triadic structure became even more prevalent.
Throughout the Medieval period, predominantly monophonic Gregorian chant was used in sacred music, although polyphonic music began to develop towards its later part. But it was with the transition into the Baroque era that the triadic harmony we recognize today took its position as the foundation of Western music.