The correct answer is (i), (ii).
(i) National income is the total income earned by all factors of production in a country in a given period of time. It is equal to the gross national product (GNP) minus depreciation. GNP is the total market value of all final goods and services produced in a country in a given period of time. Depreciation is the decrease in the value of capital goods over time due to wear and tear.
(ii) National income can also be calculated as the sum of factor incomes, which are the incomes earned by the factors of production: land, labor, capital, and entrepreneurship. Rent is the income earned by landowners. Wages are the income earned by workers. Salaries are the income earned by salaried employees. Interest is the income earned by lenders. Profit is the income earned by entrepreneurs.
(iii) Variable cost is the cost of production that changes as the level of production changes. It includes the cost of raw materials, labor, and energy. Average variable cost is the variable cost per unit of output. It is calculated by dividing the variable cost by the level of production.
Therefore, the only two options that are correct are (i) and (ii).