The correct answer is B) oil soaked rags or rubbish. Oil soaked rags or rubbish are the chief source of spontaneous combustion aboard tankers due to the risk of self-heating and ignition. The porous nature of these materials allows them to absorb and retain oil, which can then oxidize and generate enough heat to cause ignition, even without an external ignition source. This risk is well-recognized in maritime safety regulations, which require proper storage and disposal of such materials to prevent this type of fire hazard. The other options (A, C, and D) are incorrect because the cargo itself, when properly handled and stored, does not pose the same risk of spontaneous combustion as oil-soaked materials. Kerosene, gasoline, and JP4 are the cargo, not the source of spontaneous combustion.