Which of the following is an example of gene therapy?

Producing polio vaccine. Production of pest resistant crops. Introduction of gene for adenosine deaminase in people suffering from ADA deficiency. Injecting genetically engineered insulin in people suffering from diabetes.

Introduction of gene for adenosine deaminase in people suffering from ADA deficiency.

Gene therapy is defined as insertion of genes into a person's cells or tissues to treat diseases specially genetic or hereditary diseases.