SCID, a disease that can be cured by gene therapy is due to the deficiency of

The correct option is A ADA enzyme
ADA-SCID results from mutations in the ADA gene, which provides instructions for producing the ADA enzyme. This enzyme is found throughout the body but is most active in lymphocytes, which are specialized white blood cells that protect the body from potentially harmful invaders by making immune proteins called antibodies or by directly attacking infected cells.