Which of the following best describes a toxin?

A toxin is defined as a poison that is produced by living organisms. This includes substances created by animals, plants, and microorganisms that can cause harm to other living beings when introduced into their systems. Toxins can be natural compounds that serve as a defense mechanism for the producing organism against predators, pathogens, or competition.

In contrast, the other options describe substances that do not fit this definition. An artificial pesticide is a man-made chemical designed to control pests but is not produced by living organisms. A harmless chemical does not pose a threat to health, which is contrary to the nature of a toxin. A synthetic fertilizer, while it may be chemically active, is not a poison produced by a living organism; rather, it is designed to promote plant growth. Thus, the identification of a toxin as a substance created by living entities is what solidifies option B as the best description.