Which of these situations would have been considered blacklisting?

I believe the answer is: A manager refuses to hire a worker who is in favor of unions

blacklisting refers to the actions made by entities to avoid a certain group of people which are in a similar situation.

Union workers are known to cause problems for employers and disrupt the working schedule that is set by managers. So, employers in industries tend compile union workers name into a list and share it with each other so they can prevent them from obtaining jobs.