Final answer:
Interest payments on the national debt is the largest category of federal spending, comprising a substantial portion of the federal budget.
Explanation:
Interest payments on the national debt is the largest category of federal spending among the options provided. As indicated in various data sets and figures, including 2014 and 2019 statistics, this spending category, along with national defense, Social Security, and healthcare, consistently accounts for a significant portion of the federal budget, usually around 60-73%.
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