Which of the following is the simplest living organism?

Among the given options, the simplest living organism is a virus. Viruses are tiny, infectious agents that are not technically alive because they cannot replicate or carry out metabolic activities without a host cell. They consist of a genetic material (DNA or RNA) enclosed within a protein coat, known as a capsid. Some viruses also have an additional outer envelope made of lipids. Viruses rely on host cells to replicate and reproduce, hijacking the cell's machinery to make copies of themselves. Once inside a host cell, the virus injects its genetic material and takes over the cell's metabolism to produce new virus particles. These particles can then infect other cells and continue the cycle of infection. In contrast, the other options, such as Paramecium, Amoeba, and Chlamydomonas, are all considered unicellular organisms because they are made up of a single cell and have the ability to carry out metabolic activities and reproduce on their own. They also have more complex structures and mechanisms than viruses, such as organelles and a range of cellular processes.