Understeer is likely to occur under which circumstances?

Understeer occurs when the front wheels of a vehicle lose grip while attempting to steer, resulting in the car continuing in a straight line rather than following the intended curve. This phenomenon is particularly prevalent in front-wheel drive vehicles, especially when they are driven at excessive speeds.

When a front-wheel drive car is taken into a corner at too high a speed, the front tires can reach their limit of adhesion. The loss of traction in the front wheels means the driver struggles to maintain control, leading to understeering. This scenario is exacerbated by the car's weight distribution and the dynamics of how front-wheel drive systems operate, where the engine's weight is over or near the front wheels.

In contrast, driving at moderate speeds or in a rear-wheel drive vehicle typically offers more control over the steering and can reduce the likelihood of understeer. Similarly, the presence of passengers influences the weight distribution but does not inherently cause understeering. Therefore, the answer highlighting excessive speed in a front-wheel drive car accurately identifies the conditions that contribute to understeer.