Which one of the following statements is correct?

Option 3 is correct. Viroids are infectious plant pathogens composed solely of a small, single-stranded circular RNA molecule and lack a protein coat; they reproduce only within host cells [1]. Option 1 is false because prions are infectious proteins that contain no nucleic acid and are not cells. Option 2 is incorrect: Mycoplasmas characteristically lack a rigid cell wall (they have sterols in their membrane instead), so their “cell wall” cannot be composed of amino sugars like peptidoglycan. Option 4 is false because rickettsiae are obligate intracellular, Gram-negative bacteria and thus possess a bacterial cell envelope consistent with Gram-negative organisms, not a complete absence of cell wall.

Sources

1. [1] https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/General_Biology_(Boundless)/21%3A_Viruses/21.04%3A_Prions_and_Viroids/21.4.01%3A_21-4A-_Prions_and_Viroids