Final answer:
Viruses are atypical to the cell theory as they lack cellular structures, cannot function metabolically on their own, and only replicate within host cells.
Step-by-step explanation:
The type of cell that is atypical to the concept of cell theory and doesn't quite fit within the definitions of either prokaryotic or eukaryotic cells is c. Virus. Cell theory states that all living organisms are composed of cells, and that cells are the basic unit of life. Viruses challenge this concept because they do not have a cellular structure, cannot carry out metabolic processes on their own, and only replicate within the host cells they infect. Unlike eukaryotic cells which have a well-defined, membrane-bound nucleus, or prokaryotic cells like bacteria that have DNA but no nucleus, viruses are not cellular and lack many structures common to cellular life, such as ribosomes and the means to generate energy.