During which condition might a vehicle's tyres offer the least grip?

The scenario where a vehicle's tyres offer the least grip occurs when the road is wet and the tyres are worn. Wet conditions significantly reduce the friction between the tyres and the road surface. When tyres are worn, their tread pattern becomes less effective at channeling water away, increasing the potential for hydroplaning and drastically reducing grip. This combination of wet roads and worn tyres can lead to longer stopping distances and decreased control, making it a critical condition for drivers to be aware of.

In contrast, dry weather paired with new tyres generally enhances grip due to the optimal tread depth and fresh rubber making contact with the road. When driving in a straight line, the forces acting on the tyres are relatively straightforward, which typically doesn’t limit grip significantly. Lastly, although downhill slopes can influence control and increase the risk of skidding due to gravity, the primary issue with tyre grip arises in the presence of wet conditions combined with tyre wear. Therefore, the greatest risk to grip is indeed found in scenario A.