Explanation:

- Option 1: not included in national income

- Household inventory isn't part of national income calculations.

- National income focuses on production by firms, not household storage.

- Option 2: a stock concept

- Stock concepts represent quantities at a specific time.

- Household inventory is the sum of goods at a specific moment.

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

- This combines the first two options.

- Household inventory isn't in national income and is a stock concept.

- Option 4: none of these

- This suggests that neither statement is true, which is incorrect.

Correct Answer: Option 3: both (a) and (b)