Compared to bacteria and normal cells, virus particles are nothing alike. Virus particles are about one-millionth of an inch (17 to 300 nanometers) long. Viruses are about a thousand times smaller than bacteria, and bacteria are much smaller than most human cells. Viruses are so small that most cannot be seen with a light microscope, but must be observed with an electron microscope.

That's why the correct answer is Virus.

The smallest microorganism is a virus.

Explanation:

Definition of microorganism:
A microorganism is a living organism that is too small to be seen with the naked eye. They can only be seen under a microscope.

Comparison of the given options:

- Algae: Algae are a large and diverse group of photosynthetic organisms. They can be single-celled or multi-celled. They are larger than bacteria, protozoa, and viruses.

- Bacterium: Bacteria are single-celled microorganisms that can be found in various environments. They are smaller than algae and protozoa but larger than viruses.

- Protozoa: Protozoa are single-celled eukaryotic microorganisms. They are larger than bacteria and viruses.

- Virus: Viruses are the smallest microorganisms. They are much smaller than bacteria, protozoa, and algae. Viruses are not considered to be living organisms because they cannot replicate or carry out metabolic processes on their own. They can only reproduce inside the cells of other organisms.

Characteristics of viruses:

- Structure: Viruses are composed of genetic material (DNA or RNA) surrounded by a protein coat called a capsid. Some viruses also have an outer envelope made of lipids.

- Reproduction: Viruses cannot reproduce on their own. They can only replicate inside the cells of other organisms, called host cells. They hijack the host cell's machinery to produce more viruses.

- Size: Viruses are extremely small, ranging in size from about 20 to 400 nanometers. They are smaller than most bacteria and cannot be seen with a light microscope. Electron microscopes are required to visualize viruses.

- Host specificity: Viruses are highly specific to their host organisms. Each virus can only infect certain types of cells within specific host species.

- Disease-causing: Many viruses are pathogenic and can cause diseases in humans, animals, and plants. Examples of viral diseases include the flu, HIV/AIDS, and COVID-19.

Conclusion:

Among the given options, the smallest microorganism is a virus. Viruses are much smaller than bacteria, algae, and protozoa. They are composed of genetic material surrounded by a protein coat and require a host cell to reproduce. Viruses are highly specific to their host organisms and many of them can cause diseases.