A normal functional copy of the human gene coding for ADA is inserted into these cells using a retroviral vector after being removed from the patient's bone marrow. The patient's bone marrow is then given the cells that have undergone this treatment. These cells create functioning ADA genes in their lymphocytes, which reactivate the immune system of the victim. Though these cells are not immortal, the patient needs regular infusions of these genetically modified lymphocytes, Hence, Option is correct.

Option is not the correct answer as the lymphocytes from patient's blood are grown in culture, outside the body but it is not the correct reason.

In option , if the gene isolated from bone marrow cells producing ADA is introduced into cells at early embryonic stages, it could be a permanent cure

Option is just a process in periodic infusion but not the reason.