Which of the following is a clue that indicates a potential collision when passing parked vehicles?

The correct choice is the presence of a driver sitting behind the wheel of a parked vehicle. This situation serves as a significant clue that indicates the potential for a collision. A driver may suddenly decide to pull out into traffic without prior warning, which can pose a danger to anyone passing the vehicle. Since the parked vehicle could begin to move unexpectedly at any time, it's vital for drivers to remain vigilant and maintain a safe distance when passing by.

In contrast, flashing lights from a parked vehicle typically signal that the vehicle may be stopped for a reason, such as emergency or hazard situations, which may necessitate caution but does not inherently suggest movement. Traffic cones near the roadway indicate construction or alteration to the usual driving path, prompting alertness, but they do not directly relate to the behavior of parked vehicles. The sound of an approaching ambulance is a crucial auditory cue for all road users, yet it does not specifically alert drivers regarding the parked vehicles' status or the behavior of their occupants. Thus, the person behind the wheel is the most direct indicator of potential risks in this context.