Which of the following organisms is NOT classified as a microorganism?

The classification of organisms into categories such as microorganisms and larger life forms is based on their size, complexity, and characteristics. Microorganisms are typically defined as organisms that are microscopic and can only be seen with a microscope. This group includes bacteria, viruses, fungi (such as yeast), and certain protozoa.

In this context, mammals, which include all species of mammals from humans to whales, are complex organisms that are made up of numerous cells organized into tissues and organ systems. They are large enough to be seen with the naked eye and possess characteristics like being multicellular, having differentiated tissues, and typically reproducing sexually.

The correct answer identifies that mammals do not fall under the category of microorganisms due to their size and level of complexity, contrasting with bacteria, viruses, and yeast, all of which are classified as microorganisms due to their microscopic nature and simplicity.