Final answer:
In heavy fog, use emergency flashers and keep headlights on to warn other drivers that you are not moving.
Step-by-step explanation:
To warn other drivers that you are not moving in heavy fog, you should use emergency flashers. This will make your vehicle more visible to other drivers and alert them to the fact that you are stationary. Using headlights is also important to keep on, as it helps others see you better in low visibility conditions. Periodically sounding the horn may not be effective in this situation and shutting headlights off would make you less visible to other drivers.