Question

the four main classes of microorganism are bacteria, fungi, viruses, and what?
pathogens
toxins
microbes
parasites

Explanation:

Microorganisms are tiny living organisms. The four main classes are bacteria, fungi, viruses, and parasites. Pathogens are disease - causing agents which can include some from the main classes. Toxins are harmful substances, not a main class of microorganisms. Microbes is a general term for microorganisms. Parasites are one of the four main classes along with the others mentioned.

Answer:

D. Parasites