What can cause unnecessary obstructions?

In German traffic regulations, unnecessary obstructions on the road can lead to increased congestion and potentially dangerous situations. The correct combination includes all three scenarios you've mentioned.

1. Stalling the engine in dense traffic creates sudden stops, disrupting the flow of vehicles and possibly leading to accidents.
2. Vehicles proceeding too slowly without valid reasons can obstruct faster traffic, causing frustration and potential safety hazards.
3. Motor vehicles that break down due to running out of fuel represent a predictable issue; drivers are expected to manage their fuel levels responsibly. A vehicle stopped on the road due to lack of fuel can create significant obstructions, especially in high-traffic areas.

Addressing all these scenarios is crucial for maintaining smooth traffic flow and ensuring safety on the roads. Each factor contributes to possible obstructions, and recognizing them helps drivers understand the importance of being proactive in preventing traffic issues.