Pavement is most slippery in a rainstorm...

The correct answer highlights that pavement is most slippery just after the storm starts due to the initial mixing of rain with oil and other substances that have accumulated on the roadway. When it begins to rain, the first few drops of water lift these substances to the surface, creating a very slick and hazardous driving condition. This combination of water and oils creates a film that can significantly reduce traction for vehicles, increasing the risk of skidding or losing control.

As the rain continues, the water helps to wash away some of the oil and debris, which can improve traction over time. Therefore, the danger decreases as the storm progresses, making the initial moments of rainfall the most treacherous for drivers.