what tactic did the pinkertons use to bust unions?\nunion banning\nspying\nblacklisting\nbribery

Brief Explanations:

The Pinkertons were a private detective agency. They were known for using espionage - like tactics against unions. Spying on union activities and members was a common way they tried to undermine unions. Union banning was not a direct Pinkerton tactic. Blacklisting was more of an employer - led action in some cases and not the main Pinkerton method. Bribery was not their most well - known union - busting tactic.

Answer:

spying