The correct answer is option 4, "sale of land." National income is a measure of the total value of goods and services produced by a country over a specific period of time. It includes income from various sources such as salaries, wages, profits, and taxes.

Option 1, "sales of a firm," is included in national income because it represents the value of goods and services produced by the firm.

Option 2, "salary of employees," is also included in national income as it represents the income earned by individuals through their employment.

Option 3, "exports of the IT sector," is considered as a part of national income, as it reflects the value of goods and services produced by the IT sector and sold abroad, which contributes to the overall economic output of the country.

However, option 4, "sale of land," is not considered in national income. While the sale of land may lead to income for the seller, it is not included in national income because it does not represent the production of goods or services. Instead, it reflects a transfer of ownership of a physical asset.