Among the options provided, the Pure Food and Drug Act had a significant impact on the economy as part of Theodore Roosevelt's Square Deal. This legislation aimed to regulate and ensure the safety and quality of food and pharmaceuticals, which not only protected consumers but also fostered trust in the market. The establishment of standards for food and drug safety contributed to the growth of the food and pharmaceutical industries by ensuring that products were safe for consumption.
The 16th Amendment, which allowed for federal income taxation, also had a major long-term economic impact, but it is not specific to the Square Deal itself.
While the Wisconsin Idea and changes to the Interstate Commerce Commission laws were important reforms, they did not have as direct an economic impact as the Pure Food and Drug Act. Thus, the Pure Food and Drug Act is the best choice in terms of immediate and significant economic impact related to the Square Deal.