Final answer:
Passengers should stow their carry-on luggage under the seat in front of them or in an overhead bin when the captain turns on the fasten seat belt sign.
Step-by-step explanation:
When the captain turns on the fasten seat belt sign, it means that passengers should secure their carry-on luggage. A) It is not recommended to keep your carry-on luggage on your lap as it may cause inconvenience and potential safety hazards. B) The best option is to stow your carry-on luggage under the seat in front of you, as this keeps the aisle clear and ensures easy access to it. C) Alternatively, you can place your carry-on luggage in an overhead bin at any time, as long as it is securely stored.