Under which condition might spontaneous heating occur?

Spontaneous heating refers to the phenomenon where a material increases in temperature due to internal chemical reactions, eventually reaching a point where it ignites without an external ignition source. This process is often associated with the oxidation of organic materials, such as oils, sawdust, or compost, in the presence of oxygen.

When considering the correct condition for spontaneous heating, the presence of a mixture of fuels along with oxygen is crucial. The various fuels can contribute to exothermic reactions, where the heat produced can raise the temperature of the mixture over time. The individual fuels can interact, and if the heat generated is not dissipated effectively, it can lead to a rise in temperature sufficient for ignition.

For instance, in compost piles, as microorganisms break down organic material, heat is generated. If the pile is too compact or insulated, the heat accumulates and may eventually cause spontaneous combustion.

The other conditions mentioned, such as being limited to dry environments, only occurring during the day, or requiring flames to be present, do not accurately represent the fundamental requirements for spontaneous heating. Spontaneous heating can occur in a variety of environments, including humid ones, and does not need to be daylight or have flames present at any stage.