Which of the following is an example of a toxin?

Ricin is classified as a toxin because it is a naturally occurring protein that can be extracted from the seeds of the castor oil plant. It is highly potent and can inhibit protein synthesis in cells, leading to cell death. The severity of its effects depends on the dose and route of exposure, making it a concern for biological safety and terrorism.

In contrast, Brucellosis is an infectious disease caused by bacteria, specifically the Brucella species, and does not fall under the category of toxins. Cyanogen chloride is a toxic compound but is a chemical warfare agent rather than a biological toxin. Similarly, Ebola refers to a viral disease caused by the Ebola virus, which is also not a toxin but an infectious agent that can cause severe hemorrhagic fever in humans. Thus, ricin is the clear example of a toxin among the given choices.