Explanation:

The correct option is (A) Prion. Prions are unique infectious agents composed solely of misfolded proteins, lacking any genetic material (DNA or RNA). They cause transmissible spongiform encephalopathies. Option (B) Virion refers to a complete, infectious virus particle, which contains nucleic acid (DNA or RNA) encased in a protein coat. Option (C) Virus is a general term for an infectious agent that also contains nucleic acid. Option (D) Virusoids are subviral agents consisting of circular RNA molecules, thus containing nucleic acid. Therefore, only prions fit the description of a proteinaceous particle without nucleic acid.