Violence erupted at the homestead works in 1892 when: 1. andrew carnegie announced the plants sale to j. p. morgan. 2. henry frick tried to break a strike by bringing in pinkertons. 3. andrew carnegie tried to replace one union with another one. 4. police attempted to break up a protest meeting.

The correct answer is 2. henry frick tried to break a strike by bringing in pinkertons

The pinkertons were hated by the people. They were something like an independent detective agency and they were hated because they were controversial decision makers, meaning that they often resorted to police brutality or were highly corrupt or were drunkards or anything similar. When he brought them to solve the problem, the violence escalated.