Which of the following risks is associated with acetylene?

Acetylene is known for its high flammability and can pose a significant explosion hazard, particularly when mixed with air in the right proportions. When acetylene is present in an environment where it can form a mixture with air, even a small spark or heat source can lead to a rapid combustion reaction, creating the potential for a violent explosion.

This distinct characteristic of acetylene requires stringent precautions during storage and handling. It's primarily used in welding and cutting processes, where its combustion properties are beneficial, but it is essential to ensure that it is handled in a well-ventilated area and away from sources of ignition to mitigate the explosion risk.

Other options highlight potential hazards associated with other substances or scenarios, but they do not specifically apply to acetylene in the same critical way as the explosion hazard when mixed with air. For instance, although some gases may have toxic effects, acetylene's main hazard and what makes it particularly noteworthy is its explosive nature in the presence of air.