Final answer:
The correct answer to the question is 'at the start' of rainfall, since the initial mix of water with oil residue on roads causes increased slipperiness.
Step-by-step explanation:
It is usually most slippery at the start of rainfall. This is because in the beginning, the rain mixes with oil residue on the road surface, which makes it especially slick. As the rain continues, it washes away these oils, gradually decreasing the slipperiness. Therefore, option a) at the start, is the correct answer. It's similar to observing the disorganized manner in which raindrops fall; initially, conditions can be more hazardous due to the unexpected mix of water and oils before patterns establish and the surface is cleansed.