The correct option is D ssRNA not enclosed by a protein coat
Viroids are generally known as plant pathogens. They are small, single-stranded, circular RNA particles. They do not have a capsid or outer envelope of protein making them simpler than virus, but like viruses, can reproduce only within a host cell. They do not have capacity to make their own protein. They produce only a single, specific RNA molecule. Thus, the correct option is 'ssRNA not enclosed by a protein coat.'