What 215 CE event indicated that the Han Dynasty was on its way out of power?

the riot stirred up poor merchants to end taxation

the Battle for Ch'ung against Hun invaders

the Merchant's Revolt organized by silk traders

the Yellow Turban Rebellion, led by a secret Taoist Society

The Yellow Turban Rebellion led by Zhang Lu, the leader of the secret Taoist society called the Celestial Masters, began in 211 and was quelled by the Han warlord Cao Cao four years later. This revolt and the previous revolt of 184 had the effect of strengthening Han generals to the extent that they would not disband their armies nor relinquish their command over their troops, enabling them to bring the last remnants of the Eastern Han dynasty to and a few years later, in 220 AD. The empire broke down into different domains, each controlled by a warlord of the extinct Han dynasty.