Cell theory does not apply to

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Hint: These are the submicroscopic infectious agents that are considered to be alive only in host cells. These microorganisms replicate in the host cells and infect.

Complete answer:
The cell theory states that
1. All living organisms are made of cells.
2. Cells arise from pre-existing cells.
As we know viruses are those microorganisms that are alive when present in a host cell. Thus there is a lot of debate as to whether or not viruses should be considered to be alive. To survive, viruses must invade a particular host cell and utilize the host’s genetic material to reproduce(replicate). Thus we can conclude with this that Viruses are not true cells. They lack the most important part of the cell, the protoplasm. Therefore, the cell theory does not apply to viruses.

Additional Information
1. Viruses are acellular, they contain no cytoplasm or any of the cellular organelles (Golgi body, mitochondria, e, plastids, etc). They carry out no metabolism on their own (they are not semi-autonomous) and they have to replicate using the host cell's genetic material.
The vast majority of viruses possess either DNA or RNA but not both DNA and RNA.
2. Almost every fungus grows as a tubular filament called hyphae. An interwoven/intertwined mass of hyphae is known as mycelium. The walls of hyphae are made up of chitin (a polymer of N-acetylglucosamine). Fungi grow and disperse themselves by releasing spores( through sporangium),
Fungi are also heterotrophic (can't synthesize their food, depending on autotrophs).
3. Lichens show varieties of colors, sizes, and forms and are sometimes plant-like (they have root-like structures, leaf-like structures) but lichens are not true plants. Lichens may have tiny, leafless branches (fruticose), flat leaf-like structures (foliose), or other growth forms.
4. Algae are photosynthetic. They can be either unicellular or multicellular organisms.
Algae lack a well-defined body, instead, they have a thallus.
Structures like roots, stems, or leaves absent in algae. Algae are found mostly in aquatic moist places. Algaes perform both asexual and sexual modes of reproduction.

So, the correct answer is ‘Virus’.

Note: Algae and lichen both are prokaryotic organisms, fungi is a unicellular eukaryotic organism and viruses are neither prokaryotic organisms nor eukaryotic organisms.