Answer:
The event that had a negative impact on the power and popularity of the Knights of Labor was the Haymarket Riot.
Explanation:
The Haymarket Riot was a historical event that took place in Haymarket Square, Chicago, on May 4, 1886, that was the high point of a series of protests that since May 1 had occurred in support of the workers on strike, to claim the 8-hour workday. During a peaceful demonstration an unknown person threw a bomb at the police trying to dissolve the act violently. This led to a trial, years later described as illegitimate and deliberately malicious towards eight anarcho-communist workers, where five of them were sentenced to death (one of them killed himself before being executed) and three were detained.
