What is the greatest danger for a motorcycle rider when passing parked cars?

Answer: A — car pulling out

The greatest danger for a motorcycle rider when passing parked cars is the risk of a parked car's door opening. This situation presents an immediate hazard because a door can swing out unexpectedly into the path of a passing motorcycle. Since motorcycles are narrower than cars, they might be easily caught off guard by a suddenly opened door, leading to a potentially serious collision. Motorcycle riders must always be alert and maintain a safe distance when passing parked vehicles to avoid this common but dangerous scenario. The other options do present risks, but the sudden and unpredictable nature of a car door opening creates a particularly acute threat that riders must actively guard against. While a car pulling out or road debris can also pose risks, they are relatively more predictable compared to the abrupt nature of an opening door from a parked car.