Explanation:
The first clinical gene therapy was done for the treatment of SCID (Serve Combined Immuno Deficiency resulting form deficiency of ADA).The first clinical gene therapy was given in 1990 to a 4-years old girl with adenosine deaminase (ADA) deficiency. This enzyme is very important for the immune system to function. ADA deficiency can lead to severe combined immune deficiency (SCID).Hence, the correct option is (D).