Final answer:
The piece of legislation that was declared unconstitutional was the National Industrial Recovery Act (NIRA). This act was part of the New Deal but was struck down by the Supreme Court, which led to the later passage of the National Labor Relations Act to replace its labor provisions.
Step-by-step explanation:
Among the options listed, the piece of legislation that was declared unconstitutional by the Supreme Court is d. National Industrial Recovery Act (NIRA). This act, part of the New Deal legislation, included provisions to recognize the rights of workers to unionize and was intended to address the economic and labor crisis of the Great Depression. However, it was struck down because the Supreme Court found that it gave the executive branch regulatory powers that exceeded what the Constitution allowed, particularly in affecting local commerce that did not involve interstate trade.
The National Labor Relations Act, also known as the Wagner Act, was passed to reaffirm and replace those parts of the NIRA related to labor practices after it was declared unconstitutional. The Wagner Act successfully withstood constitutional challenges and was upheld by the Supreme Court.
The other acts listed, including the Clayton Act, Taft-Hartley Act, and the National Labor Relations Act, were not declared unconstitutional and so do not fit the description of the act in question.