Which of the following organisms is an example of a microorganism?

Bacteria are classified as microorganisms due to their microscopic size and unicellular structure. They are among the simplest and most abundant forms of life on Earth and can only be observed with a microscope, illustrating the defining characteristics of microorganisms.

In contrast, horses, trees, and worms are larger, multicellular organisms that are not classified as microorganisms. These organisms can be seen without the aid of a microscope and possess complex structures and systems that differentiate them from microorganisms like bacteria.