The correct option is E Normally reproduce only in host cells
According to the theory, all living organisms are composed of cells and cell products, which are the smallest entities that can be called as 'living'. Cells are the functional unit of an organism. All living cells arise from pre-existing cells i.e. cells have the ability of division. Viruses are the acellular structures that have nucleic acid and the outer protein coat but lack DNA replication machinery and replicate and reproduce inside the host cell using the replication machinery of the host only. This makes them an exception to cell theory as all living organisms are capable of DNA replication and reproduction by themselves. Thus, the correct answer is option E.