Nucleic acid does not occur in

Prions are misfolded proteins which cause several fatal neurodegenerative diseases in animals and humans. In humans, prions cause Creutzfeldt Jakob disease (CJD) and in cattle, it causes mad cow disease. Prions are only proteins and do not include nucleic acids. Viroid is composed of single-stranded RNA without a protein coat. Mycoplasma is a bacterium with DNA. Viruses are small infectious particles made of nucleic acids and a protein coat. So, the correct answer is 'Prion'.