Answer:
The smallest option among the choices given is A. Virus.
Step-by-step explanation:
Virus is an infectious agent that is much smaller than a typical prokaryotic or eukaryotic cell. It is essentially a small package of genetic material (either DNA or RNA) enclosed in a protein coat called a capsid. Some viruses also have an outer envelope made of lipids.
A prokaryotic cell, on the other hand, is a unicellular organism that lacks a nucleus and other membrane-bound organelles. Examples of prokaryotic cells include bacteria and archaea. While they are small, prokaryotic cells are generally larger than viruses.
Eukaryotic cells are more complex than prokaryotic cells, with a true nucleus and various organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Eukaryotic cells are generally larger than prokaryotic cells.
Finally, human beings are multicellular organisms composed of trillions of eukaryotic cells. Therefore, humans are much larger than any individual cell or virus.