Which of the following biological agents is a pathogen?

Bacteria are indeed classified as pathogens because they are microorganisms that can cause disease in a host organism. Pathogens are agents that can induce illness or disease, and bacteria fit this definition as many species are capable of triggering infections, ranging from mild to severe.

In contrast, plastic and metal do not fall under the category of biological agents, as they are non-living materials that do not have the capability to cause disease on their own. Chemical substances can sometimes be toxic and lead to harmful effects, but they are not classified as biological pathogens in the same way that bacteria are. Thus, the unique characteristic of bacteria as living microorganisms capable of causing diseases confirms why this choice is the correct answer.