When does a bus have the most traction while backing up?

The correct answer highlights that a bus has the most traction while backing up when the wheels are rolling just short of locking up. When wheels are in this state, they maintain the best contact with the road surface, which is crucial for providing adequate traction. Locking up the wheels can lead to skidding, resulting in a loss of control and the inability to steer effectively. Conversely, if the wheels are merely spinning without sufficient grip--as would occur if the surface is slick or the brakes engage suddenly--it significantly reduces control and stability.

Reversing slowly is also a good practice for maintaining control, but it is the condition of the wheels themselves that primarily determines traction. Warmer temperatures do not inherently affect traction unless they pertain to specific conditions, such as road surface materials reacting to heat, but they are not a fundamental factor in traction when backing up.