Gene rapy was first used in treatment of

Gene therapy is the process of introduction of DNA into living human beings in order to treat disease. It is used to replace a missing gene product or to correct mutant alleles. The first human gene therapy trial was conducted at the National Institutes of Health in Bethesda, Maryland, in 1990 on a four-year-old girl suffering from ADA deficiency that causes SCID. A functional ADA gene was introduced ex-vivo in bone marrow cells of the child through an engineered retrovirus containing a functional ADA gene. The treated cells were reintroduced into the patient?s marrow. 

Thus, the correct answer is option C.