What should flight attendants instruct passengers to do if there is smoke in the cabin?

When there is smoke in the cabin, the appropriate instruction for passengers is to stay low and cover their mouths with clothing to avoid inhalation. This is due to the fact that smoke rises, and the air near the floor is typically cleaner and safer to breathe. By getting low, passengers can minimize their exposure to harmful smoke and increase their chances of survival. Covering their mouths with clothing also acts as a filter, helping to reduce the amount of smoke inhaled.

The other options do not effectively address the immediate danger posed by smoke. Remaining seated with seat belts fastened can be appropriate in various situations but doesn't assist in protecting passengers from smoke inhalation. The use of oxygen masks is crucial in high-altitude emergencies or when there is a lack of oxygen, not primarily for smoke-related incidents. Opening windows isn't possible on an aircraft and could introduce more hazards, so it is not a viable safety measure in the case of cabin smoke.