In which of the following cases should a driver flash the headlights of their car?

Flashing the headlights of a car serves as an important communication tool between drivers, particularly for warning others of your presence. This is especially relevant in situations where visibility is poor, such as at dusk, dawn, or during inclement weather. By flashing your headlights, you can make other road users more aware of your vehicle and potentially prevent accidents, especially in scenarios where they might not see you due to obstacles or lack of lighting.

Choosing to flash headlights to signal other drivers to change lanes, indicate a parked car, or alert pedestrians at crosswalks does not align with common driving practices. Using headlights in these contexts may not effectively convey the intended message and may lead to confusion among other road users. For instance, lane changes are typically signaled using turn indicators, while parked vehicles do not require such signaling. Pedestrian awareness is usually managed through other means, such as stopping or yielding, rather than flashing headlights. Thus, the option highlighting warning others of your presence is the most appropriate use of this action.