National Income is correctly defined as the Net National Product at factor cost (Option 2), which means the total monetary value of all final goods and services produced within the boundary of a nation in a year, evaluated at the factor cost (cost of the resources used to produce those goods and services).

Option 1, Net National Product at market price, is incorrect because it includes not just the factor cost but also indirect taxes which are not part of the National income.

Option 3, Net Domestic Product at market price, and Option 4, Net Domestic Product at factor cost, are both incorrect because they refer to the value of goods and services produced within a country`s geographical boundaries, excluding the foreign income of its residents, which is accounted for in the Net National Product. Therefore, Options 3 and 4 do not give a complete picture of the National Income as they do not account for the incomes earned by nationals from overseas investments.