When driving through heavy fog, you should:

Driving through heavy fog significantly reduces visibility, which can increase the risk of accidents. Slowing down is essential because it gives you more time to react to any obstacles, changes in traffic conditions, or the sudden appearance of other vehicles. Reduced speed in foggy conditions allows for better control of the vehicle and enhances your ability to notice warning signs or other vehicles that may be difficult to see.

While other options might seem plausible, they do not enhance safety in situations with poor visibility. For instance, using bright lights can cause glare that reflects off the fog, making it harder to see. Following closely behind another vehicle can limit your reaction time if they suddenly stop or slow down. Turning on parking lights does not provide sufficient visibility for driving and may confuse other drivers about your intentions. Therefore, reducing speed is the most effective strategy for navigating through heavy fog safely.