The equilibrium of national output is determined by:

The equilibrium of national output is determined by the intersection of aggregate demand and short-run aggregate supply. In economic terms, the aggregate demand curve represents the total demand for goods and services in an economy at various price levels, while the short-run aggregate supply curve reflects the total supply of goods and services that producers are willing and able to sell at those price levels in the short run.

When these two curves intersect, it indicates a point where the quantity of goods and services that consumers are willing to buy (aggregate demand) matches the quantity that producers are willing to sell (short-run aggregate supply). This equilibrium reflects the prevailing price level and output level in the economy. Any shift in either curve can lead to changes in output and price levels, highlighting the dynamic nature of the economy.

The other choices do not capture the complete picture of equilibrium determination. Aggregate demand alone does not account for how supply affects the output level, while short-run aggregate supply alone overlooks the demand factors necessary for equilibrium. Long-run aggregate supply is more focused on potential output in the long term and does not represent the immediate balance between demand and short-run supply. Thus, the intersection of both demand and short-run supply provides a comprehensive understanding of national output equilibrium.