Question

Companies attempted to intimidate union organizers by

blacklisting them.
suing them.
boycotting them.
imprisoning them.

Answer 1

By Blacklisting them

Answer 2

Companies attempted to intimidate union organizers by BLACKLISTING THEM.
Union organizers are those employed workers who encourage and organize unions in the work place in order to engage the employers to improve the working conditions of the workers. This type of people are usually blacklisted by employers. This is done by listing and circulating their names among employers, so that no employer will employ them.