**Introduction:**

National income is the total value of goods and services produced within a country's borders in a specific time period, usually a year. It is an important economic measure that helps in understanding the overall economic performance of a country. The components of national income include various sources of income generated within the domestic economy.

**Components of National Income:**

1. **Mixed Income:** It refers to the income earned by self-employed individuals who run unincorporated businesses or work in partnerships. This includes profits, wages, and other forms of compensation received by individuals engaged in activities such as farming, small-scale businesses, and freelance work.

2. **Net Factor Income from Abroad:** This component represents the income earned by domestic residents from their investments and work in foreign countries, minus the income earned by foreign residents from their investments and work within the domestic economy. It includes income from factors of production such as wages, salaries, profits, and dividends.

3. **Operating Surplus:** Operating surplus, also known as corporate profits, refers to the surplus earned by corporations after deducting their costs of production, including wages, rent, interest, and taxes. It represents the portion of national income that accrues to the owners of capital and reflects the profitability of businesses.

4. **Depreciation:** Depreciation is the reduction in the value of fixed assets due to wear and tear, obsolescence, or any other cause. It represents the capital consumption or the decline in the value of physical assets used in the production process. Depreciation is an important component in calculating net national income, as it accounts for the wear and tear of capital stock.

**Answer:**

The component that is not a part of national income is **Depreciation**. While depreciation is a crucial factor in determining net national income, it is not considered a part of national income itself. Depreciation reflects the decline in the value of physical assets and is accounted for when calculating the net value of national income.

**Explanation:**

Depreciation is excluded from national income because it represents the capital consumption or reduction in the value of fixed assets. It is a non-recoverable expense that occurs over time due to the wear and tear of capital stock. By excluding depreciation, the national income accounts for the net value of goods and services produced, without considering the decline in the value of physical assets.

Including depreciation in national income would lead to double counting, as it is already accounted for in the operating surplus component. Operating surplus represents the surplus earned by businesses after deducting their costs, which includes depreciation expenses. Therefore, depreciation is indirectly accounted for in the national income through the operating surplus, but it is not considered a separate component.

In conclusion, while mixed income, net factor income from abroad, and operating surplus are all components of national income, depreciation is not considered a part of national income itself. Depreciation is essential in calculating net national income, but it is excluded to avoid double counting and to reflect the value of goods and services produced within a country.