Correct Option (c)
Viroids are subviral pathogens composed solely of a short, circular, single-stranded RNA molecule. Unlike viruses, they lack a protein coat (capsid). They primarily infect plants, causing various diseases, and replicate within the host cell's machinery.
Incorrect Options
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Prions are infectious proteinaceous particles devoid of nucleic acids. They are not cellular organisms and thus cannot be considered free-living cells. They are known to cause transmissible spongiform encephalopathies. Mycoplasma species are generally recognized as the smallest free-living cells.
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Mycoplasma species are distinguished by the complete absence of a cell wall. This structural characteristic makes them resistant to antibiotics like penicillin, which target peptidoglycan synthesis in bacterial cell walls.
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Rickettsias are obligate intracellular parasitic bacteria that possess a cell wall. While their cell wall structure may differ from typical Gram-negative bacteria, it is present and essential for their integrity.