Appropriations specify what aspect of federal funding?

Appropriations in the context of federal funding are crucial as they define the specific amount of funds that Congress has allocated to federal agencies and programs for expenditures. This allocation process allows the government to manage its finances and ensures that funds are available for specific purposes as decreed by various laws and budgets. Appropriations serve as the legal authority for federal spending within a defined timeframe, typically a fiscal year, making them essential for efficient budget management and accountability.

The other choices do not accurately reflect the essence of appropriations: taxes pertain to revenue generation, mandatory requirements refer more to compliance and regulations that agencies must follow, while guidelines for revenue generation would relate to the overall fiscal policy rather than the specific spending authority that appropriations signify. Thus, option B's focus on the specific amount of funds allocated for spending aptly captures the critical function of appropriations in fiscal governance.