Which of the following is a non-explosive but potentially harmful agent?

Botulism is indeed a non-explosive but potentially harmful agent because it is caused by the toxin produced by the bacterium Clostridium botulinum. This toxin can lead to severe illness or death if ingested in contaminated food. Unlike the other choices, botulism does not involve an explosive reaction, making it distinct in this context.

Chlorine is a toxic gas and can be harmful through inhalation or contact but is not classified as a biological agent like botulism. Phosgene, while also a harmful gas, is associated with industrial accidents and chemical warfare rather than biological toxicity. Plutonium, a radioactive element, poses dangers primarily through its radiological properties and potential for nuclear fission but does not represent a biological toxin.

In summary, botulism stands out as the only biological agent among the options, highlighting its classification as a non-explosive but potentially fatal toxin.