Which of the following is a key action to take when driving in heavy rain?

Reducing speed and using your headlights are crucial actions when driving in heavy rain due to the significantly reduced visibility and increased stopping distance that can occur under such conditions. Slowing down allows for better control of the vehicle and provides more time to react to unforeseen circumstances, such as puddles or other vehicles.

Using headlights is equally important because heavy rain can create a moisture haze that diminishes visibility, and headlights help illuminate the road ahead and make your vehicle more visible to others. This combination of reduced speed and enhanced visibility through headlights is essential for safe driving during adverse weather conditions.

The other actions presented may lead to dangerous driving situations. For instance, attempting to increase speed to stay ahead of the rain can lead to loss of control and increase the risk of accidents. Ignoring the rain and maintaining normal speed overlooks the critical changes in driving conditions, while using the highest setting for windshield wipers may not necessarily be adequate if visibility is severely compromised—it's more effective when combined with reduced speed and appropriate use of headlights.