The correct answer is D) rags soaked in linseed oil. Rags soaked in linseed oil or other drying oils are susceptible to spontaneous combustion due to the chemical reactions that occur as the oils oxidize. This can lead to a rapid buildup of heat within the rags, potentially causing them to ignite without an external heat source. This is a common fire hazard on vessels and is specifically addressed in regulations for safe storage and handling of such materials. The other options are incorrect because they do not pose the same risk of spontaneous combustion. Dirty swabs and cleaning gear may be combustible but do not have the same propensity for spontaneous ignition. Partially loaded fuel tanks and overloaded electrical circuits present different fire hazards, but do not directly relate to the phenomenon of spontaneous combustion.