Concept:The simplest living organism is the one with the least structural complexity.
In the options given, that is the virus.
Explanation:A virus is not a complete cell.
It consists only of genetic material (DNA or RNA) surrounded by a protein coat.
It has no cytoplasm, no ribosome, and no membrane-bound organelles.
Paramecium, Amoeba, and Chlamydomonas are all unicellular eukaryotic organisms with a true nucleus and many organelles.
These cells carry out complex life processes independently.
A virus, however, can multiply only inside a living host cell.
Even though some scientists classify viruses as non-living when they are outside a host, many school biology syllabi regard them as the simplest living organisms because of their structure and ability to reproduce inside cells.
Hence, among the four choices, the simplest living organism is the virus.
Answer:B. Virus