What is a microorganism?

A microorganism is defined as a small living organism that is often too small to be observed with the naked eye and typically requires a microscope for visualization. This definition includes a variety of life forms, such as bacteria, viruses, fungi, and protozoa. Option B accurately captures this definition by stating that a microorganism is a small living plant or animal seen only with a microscope, placing emphasis on the size limitation that characterizes microorganisms in contrast to larger organisms.

In biology and microbiology, understanding the nature of microorganisms is fundamental because these entities play crucial roles in processes such as disease causation, fermentation, and ecological balance. The distinction between what can and cannot be visualized without technological assistance (like a microscope) is essential in both practical applications and theoretical studies surrounding microbiology.

Other options, while hinting at various aspects of living organisms or minerals, do not accurately define a microorganism as understood in scientific terminology. The concept of microorganisms encompasses organisms that are primarily unicellular or exist in forms that are not readily detectable without the aid of instruments designed for that purpose.