Final answer:
Pavement is most slippery at the onset of a rainstorm when oils and other substances on the ground mix with the rainwater. As the rain continues, it washes away these contaminants, making the pavement less slippery. Drivers should exercise caution during this initial period to ensure safety.
Step-by-step explanation:
Pavement is most slippery in a rainstorm just after the storm starts, which is option a. When a storm commences, oils and other substances that have not been washed away create a slippery layer. Time and continuous rainfall will eventually wash away the bulk of these surface contaminants, making the pavement less slippery as the rain persists. So, the initial period of a storm is when the pavement can be most treacherous for vehicles and pedestrians alike. It's essential for drivers to be particularly cautious during the early stages of a rainstorm to prevent accidents and maintain safety.