National income at constant prices means:

The correct answer is income estimated at base year prices.

National income at constant prices, also known as Real National Income, refers to the economic output of a country evaluated using the prices of a fixed base year. This approach adjusts the total value of goods and services for inflation or deflation, allowing for an accurate measurement of actual economic growth over time without the distorting effects of changing price levels.

In contrast, national income calculated using current year prices is called Nominal National Income, which reflects both changes in physical production and changes in market prices.