Which of the following fuels would indicate a lack of expected ignition sources?

The choice of gasoline as the fuel that indicates a lack of expected ignition sources is based on its highly flammable nature and the conditions under which it is typically ignited. Gasoline requires a significant ignition source due to its volatility and low flash point. In most environments where gasoline is present, one would typically expect to find ignition sources like open flames, sparks, or electrical equipment that could ignite its vapors.

When gasoline is used or found in a location where ignition sources are absent, this may suggest a more controlled scenario surrounding its presence, such as it being in storage. This supports the idea that the environment lacks any obvious ignition sources, which could potentially lead to a fire.

In contrast, fuels like wood, plastic, and natural gas have different characteristics and ignition behaviors. Wood can ignite easily when there is sufficient heat or a flame present, while plastic can melt and release flammable vapors that are also susceptible to ignition. Natural gas, while normally requiring an ignition source for combustion, can sometimes accumulate in areas with inadequate ventilation, leading to explosive conditions if an ignition source is introduced.

Thus, gasoline stands out as the fuel that would most strongly indicate a lack of expected ignition sources in a given environment when comparing it to the other options presented.