My adopted child feels that there is something wrong with her, and that’s the reason for her biological parents to have given her away for adoption

Reassure your child that her biological parents had to take this decision because of their unique circumstances. Reassure your child about the central position that she occupies in your life and how much you wanted to have her because of her uniqueness and specialness. Tell her that you choose her because of how wonderful she is. And that you are fortunate that she is yours.