The best answer is B.
Prions are an abnormal form of protein that has been genetically encoded.
They have no nucleic acid and are therefore very different from viruses, bacteria, fungi and other pathogens. They are quite resistant to procedures that break down nucleic acids and destroy biological forms of pathogens.
Prions cannot reproduce but they replicate by stimulating normal cellular prion protein to refold into an abnormal form called PrP Scrapie.
Prions are extremely small, even smaller than viruses.
