Answer:
The cost of building prisons would fall under discretionary spending. Discretionary spending refers to the portion of the federal budget that is not set by law and includes programs and initiatives that Congress can choose to fund or not fund each year. Building prisons is not mandated by any law, and the government has the discretion to allocate funds to build prisons or not. In contrast, mandatory spending refers to programs that are required by law, such as Social Security and Medicare, and their funding is automatically allocated based on the provisions of the law.