Final answer:
In the event of sudden fog and rapidly declining visibility, the safest course of action is to pull off the road and wait for visibility to improve.
Step-by-step explanation:
If fog suddenly descends and visibility declines rapidly, the safest action to take is to pull off the road and wait for conditions to improve. This ensures that you are not a hazard to yourself or other drivers who may not be able to see you. Attempting to sound your horn at regular intervals, increasing your speed, or reducing your speed by a fixed percentage could still result in accidents due to the unpredictable nature of other drivers' actions in poor visibility.