Option 1 is incorrect because prions are not cells at all. They are misfolded proteins that cause disease, and they aren`t capable of replicating or carrying out metabolic processes as living cells do.

Option 2 is incorrect because the cell wall of Mycoplasmas is not made up of amino sugars. In fact, Mycoplasmas lack cell walls which is why they are resistant to many antibiotics that target cell wall synthesis.

Option 3 is the correct answer. Viroids are small, circular, single-stranded RNA molecules that are capable of infecting some plants and are unique in that they do not encode any protein products. As such, their structure and functions are quite different from those of standard viruses and bacteria.

Option 4 is incorrect because Rickettsias do have a cell wall. Rickettsiae are a type of bacteria that are often associated with diseases in humans and animals. They are unique in that they are obligate intra-cellular parasites, meaning they can only survive and replicate within the cells of their host organism.