Step 1: Understanding the Concept:
The question asks to identify a "natural" factor of deterioration, as opposed to factors that are primarily human-caused or accidental events.
Step 2: Detailed Explanation:
(1) Bad storage: This is a human factor, resulting from poor management, lack of resources, or negligence.
(2) Pests: Insects (like silverfish, moths), rodents, and mold are biological organisms that exist in nature. Their infestation and the damage they cause are considered a natural process of biodeterioration. While poor storage can encourage pests, the pests themselves are a natural factor.
(3) Accidents: Accidents, such as dropping an object, are typically caused by human error. They are not a continuous natural process.
(4) Fire: Fire can be caused by natural events like lightning, but in a museum context, it is far more likely to be caused by human factors like faulty wiring, arson, or accidents. It is generally classified as a catastrophic event rather than a continuous natural factor of deterioration like pests or light.
Step 3: Final Answer:
Among the given options, pests represent a biological and natural agent of deterioration.