Final answer:
When the NO SMOKING switch is in AUTO on an aircraft, only the no smoking sign in the cabin will automatically illuminate. The fasten seatbelt sign commonly has its own switch and operational conditions.
Step-by-step explanation:
The subject in question appears to be related to aviation, specifically the operation of cabin signs. When the NO SMOKING switch is in the AUTO position on an aircraft, it is designed to control the illumination of the no smoking signs automatically. Typically, the no smoking sign will illuminate based on certain conditions such as when the aircraft is below a certain altitude, indicating that smoking is not allowed. The fasten seatbelt sign, while often also automatic, is usually controlled by a separate switch and may illuminate under different conditions such as during taxi, takeoff, and landing or when there is anticipated turbulence.
Therefore, the correct answer to the question: 'When the NO SMOKING switch is in AUTO, what signs will automatically illuminate?' is B) Only the no smoking sign in the cabin. The NO SMOKING sign's automation is typically independent of the fasten seatbelt sign in the context of typical commercial aircraft operations.