What could indicate that a driver is preparing to exit a parked vehicle?

Turning on the signal lights is a clear indicator that a driver is preparing to exit a parked vehicle, specifically on the side of the road or in a scenario where they may be merging into traffic. Signal lights are used to communicate intentions to other road users, alerting them that the driver plans to move out of the parking space or position. This action serves as a safety measure, helping to prevent accidents by making the driver's intentions known.

Other options may not provide the same level of clarity about a driver's intent. For example, turning on the vehicle’s headlights is typically done for visibility rather than signaling an exit. Opening the windows could suggest that the driver is either cooling down the vehicle or preparing to communicate with someone outside but doesn’t necessarily show an intent to exit. Using the horn could be for various reasons, like signaling to other drivers or pedestrians, but it is not specifically an indication of preparing to leave the parked position. Thus, the use of signal lights is the most relevant and recognized way to indicate an imminent action of exiting the vehicle.