The underlying principle of gene therapy is:
a. replacing a defective copy of a gene with a functional copy
b. silencing a gene so it stops producing mrna
c. making a mutation in an existing gene
d. inserting a promoter to produce large amounts of a protein

Option (A).

Explanation:

Gene therapy may be defined as the type of therapeutic medicine used to treat the genetic problems of the individual. The genetic therapy was first discovered by  Martin Cline.

In the genetic therapy, the defective copy of a gene is replaced by the function copy of gene. The gene can be inserted directly by the injections or by using any retrovirus.

Thus, the correct option is (A).

answer; i believe the correct answer is (data); /// good luck