Explanation:

- Option 1: "not included in national income"

- Household inventory refers to goods produced and retained for future use. They are not counted in national income calculation until sold.

- Option 2: "a stock concept"

- Household inventory is indeed a stock concept. It measures the quantity of goods at a particular point in time, not over a period.

- Option 3: "both (a) and (b)"

- Correct as it encompasses both explanations. Household inventory is not included in national income and is a stock concept.

- Option 4: "none of these"

- This option is incorrect as options (a) and (b) correctly describe household inventory.