Which one of the following statements is correct?

1. Statement A is incorrect. Prions are infectious agents composed primarily of misfolded proteins. They are much simpler than cells and lack the machinery for independent life; they are not free-living cells. The smallest free-living cells are generally considered to be certain bacteria, like Mycoplasmas. 2. Statement B is incorrect. Mycoplasmas are unique among bacteria because they naturally lack a rigid cell wall. Their cell membrane contains sterols, which provide some stability. They do not have a cell wall made up of amino sugars (like peptidoglycan found in most bacteria). 3. Statement C is correct. Viroids are infectious agents even simpler than viruses. They consist solely of a short strand of circular, single-stranded RNA (ssRNA) molecule, with no protein coat. 4. Statement D is incorrect. Rickettsiae are a group of bacteria (obligate intracellular parasites). Like other bacteria, they possess a cell wall (though its structure may differ slightly from typical Gram-negative bacteria). They do not lack a cell wall.