Which form of energy causes spontaneous heating of a pile of linseed oil-soaked rags?

The correct answer is based on the understanding of how spontaneous heating occurs in materials like linseed oil-soaked rags. When these rags are piled together, the linseed oil undergoes a chemical reaction known as oxidation. This process generates heat. As the heat builds up within the pile, it can reach a temperature high enough to ignite the flammable materials, leading to combustion.

Chemical energy is released during the oxidation process, and it is this release of energy that accounts for the spontaneous heating. This situation is particularly common with oils and rags, as they tend to insulate the heat and do not allow it to dissipate easily, helping the temperature to elevate rapidly.

Other forms of energy listed, such as electrical, mechanical, and radiant energy, do not directly cause the spontaneous heating in this context. Electrical energy typically involves circuits and devices, mechanical energy encompasses motion and physical work, and radiant energy relates to energy transmitted through radiation (like heat or light). In the case of the linseed oil-soaked rags, the key process involves the chemical energy linked to oxidation, making it the most relevant choice.