Final answer:
The Taft-Hartley Act included a provision requiring union leaders to sign affidavits disclaiming any Communist affiliations, among other restrictions on union activities and measures that altered labor relations.
Step-by-step explanation:
Among the actions deemed illegal under the Labor Management Relations Act (LMRA), also known as the Taft-Hartley Act, the requirement for union leaders to sign affidavits disclaiming any affiliation with Communist organizations is the one that does not fit the blank in the student's question. This specific provision was indeed a part of the act and was one of several measures that significantly altered the landscape for labor unions.
Other key provisions of the Taft-Hartley Act included the prohibition of closed shops and union shops, the limitation on secondary boycotts, and restrictions on the use of union funds in political campaigns. The act also granted presidential authority to postpone strikes that might affect national interests through a "cooling-off period", and allowed states to pass "right-to-work" laws.