The correct answer is A) careless disposal or storage of material. Spontaneous ignition can result from the improper storage or disposal of combustible materials. When flammable materials are not stored or disposed of properly, they can build up heat and eventually ignite without an external ignition source, leading to a fire. The other options, such as an unprotected drop-light bulb, smoking in bed, or worn electrical wires, can also be fire hazards, but they do not directly lead to spontaneous ignition the way careless material storage and disposal can.