Vapor lock can occur when:

Vapor lock occurs when the fuel supply line gets too hot, causing the fuel (often gasoline) to vaporize before it reaches the engine’s carburetor or fuel injector. This vaporization creates gas bubbles in the fuel line which obstructs the flow of liquid fuel. As a result, the engine can experience stalling or difficulties starting because adequate fuel is not getting to the combustion chamber. This issue is more common in hot weather or in situations where the engine generates high heat.

Other options, while they may contribute to various engine issues, do not directly lead to vapor lock. For instance, a fuel leak primarily presents a risk of fuel loss or contamination rather than vaporization caused by excess heat. Similarly, applying high pressure to the system typically helps maintain fuel flow rather than causing vapor lock. A clogged carburetor would restrict fuel flow but not primarily due to the fuel vaporizing. Thus, overheating the fuel is the key factor that leads to vapor lock.