The correct option is A Agrobacterium
Agrobacterium is considered a natural genetic engineer as it has a tendency to incorporate its genome into the plant genome. Thus, it is not used as vector in human gene therapy. Several types of viruses, including retrovirus, adenovirus, adeno-associated virus (AAV), and herpes simplex virus, have been modified in the laboratory for delivering the new gene by infecting the cell, for the purpose of gene therapy.