When is a rain-slick surface most slippery?

Answer: D — At the beginning of a rain

When a rain shower begins, the rainwater mixes with the residue and oil on the road, creating a very slippery surface. This initial period of rainfall is when the road is most slippery due to the mixture of water and contaminants on the surface, making it hazardous for riders. As the rain continues, it washes away some of the residue, improving the road's traction. Therefore, the correct answer is at the beginning of a rain.