Explanation:

- Olfactory indicators show change in:

- Smell:

- This is the correct answer. Olfactory indicators are substances that change smell when they undergo chemical changes.

- Physical state:

- Olfactory indicators do not show changes in physical state like solid, liquid, or gas.

- Color:

- These indicators do not reveal any change in color.

- Chemical state:

- They do not indicate changes in chemical state directly but through the smell.