National income equilibrium is not at the level where
The correct answer is option 'D' - Aggregate consumption is constant.

Explanation:
National income equilibrium refers to a situation where aggregate income in an economy is equal to aggregate expenditure. In other words, it is the level of income at which the total spending in the economy is equal to the total production and output.

1. Aggregate investment equals aggregate savings:
In a national income equilibrium, aggregate investment should equal aggregate savings. This implies that the total amount of money invested in the economy should be equal to the total amount of money saved by individuals, businesses, and the government. This ensures that there is no excess demand or excess supply in the economy.

2. Aggregate expenditure equals aggregate income:
In a national income equilibrium, aggregate expenditure should be equal to aggregate income. This means that the total amount of money spent by consumers, businesses, and the government should be equal to the total income earned by individuals and businesses. This ensures that there is no surplus or deficit in the economy.

3. Inflationary and deflationary gaps are absent:
In a national income equilibrium, there should be no inflationary or deflationary gaps. An inflationary gap occurs when aggregate demand exceeds aggregate supply, leading to rising prices and inflation. A deflationary gap occurs when aggregate supply exceeds aggregate demand, leading to falling prices and deflation. In a national income equilibrium, the economy operates at full employment without any inflationary or deflationary pressures.

4. Aggregate consumption is constant:
The correct answer is option 'D' - Aggregate consumption is constant. In a national income equilibrium, aggregate consumption is not necessarily constant. It can vary depending on various factors such as income levels, savings rates, interest rates, and consumer confidence. Changes in aggregate consumption can affect the equilibrium level of national income and expenditure.

In conclusion, national income equilibrium is not determined by constant aggregate consumption but rather by the equality of aggregate investment and savings, aggregate expenditure and income, and the absence of inflationary and deflationary gaps.