Explanation:

The Gross Domestic Product (GDP) is a measure of the total economic output of a country. It is calculated by summing up the value of all final goods and services produced within the country during a specific time period. However, there are certain items that are not included in GDP calculation. Let's discuss each of the given options:

1. Foods and services provided free of cost by a NGO:
NGOs (Non-Governmental Organizations) often provide goods and services to the society for free. These can include food, medical services, education, etc. Since these goods and services are not exchanged for monetary value, they are not included in GDP calculation. Including them would overstate the actual economic output of the country.

2. Housewives' work:
Housewives perform a significant amount of work in households, such as cooking, cleaning, taking care of children, etc. However, since these activities are not a part of the formal market economy and no monetary transactions are involved, they are not considered in GDP calculation. This is because GDP focuses on measuring the value of goods and services produced in the market.

3. A doctor treating his own children:
When a doctor treats his own children, there is no monetary transaction involved. The service provided is not being exchanged for money, so it is not included in GDP calculation. In order to be included, there needs to be a market transaction where money is exchanged for the service.

4. Goods in the inventory:
Goods that are produced but have not yet been sold or used are considered to be in the inventory. These goods are not included in GDP calculation because GDP only measures the value of final goods and services. Including goods in the inventory would result in double-counting, as they would already be accounted for when they are eventually sold or used.

In conclusion, GDP calculation excludes goods and services provided free of cost by NGOs, housewives' work, services provided without a monetary transaction, and goods in the inventory. These exclusions are made to ensure that GDP accurately measures the value of goods and services produced within a country's market economy.