Which of the following is the chief source of spontaneous combustion aboard tankers?

The chief source of spontaneous combustion aboard tankers is oil-soaked rags or rubbish. This is due to the nature of certain organic materials, such as oily rags, which can generate heat as they degrade. When these materials are not stored properly and are in a confined space, the heat produced can build up to a point where it leads to spontaneous ignition. This risk is particularly relevant in the marine environment where flammable liquids are present, as they can exacerbate fire hazards.

The other options, while flammable, are generally not associated with spontaneous combustion in the same manner. JP4, gasoline, and kerosene are petroleum products that require an external ignition source to ignite. They are usually stored, handled, and transported under strict safety protocols to prevent accidental ignition and enhance safety on tankers. Thus, while all these materials are flammable and present hazards, oil-soaked rags or rubbish stand out as the primary concern for spontaneous combustion.