What defines a viroid?

A viroid is indeed defined as the simplest infectious pathogen known. Viroids are composed solely of a short strand of circular RNA and lack a protein coat, distinguishing them from viruses, which have both RNA or DNA and a protein encapsidation. This minimal structure allows viroids to be significantly less complex than bacteria, viruses, and other pathogens. Their simplicity enables them to replicate within host cells, often causing diseases in plants.

The other choices illustrate concepts that do not align with the definition of a viroid. For instance, a complex multicellular organism cannot be classified as a viroid, as viroids lack cellular structure entirely. Additionally, viroids are not the largest infectious pathogens; in fact, they are among the smallest. Lastly, viroids do not function like bacteria, nor do they possess characteristics associated with antibiotic resistance, which is a feature of certain bacteria.