5) G

1 – Correct
Change in the alleles of the gene coding could have caused the change in DNA.

2 – Correct
The order of amino acids in the enzyme could have changed the protein structure of the enzyme and hence, caused the condition SCID.

3 – Correct
The order of bases in the gene coding could have changed the DNA structure.

4 – Correct
If the shape of the active site is different, then this would have prevented production of healthy white blood cells.

Since all are correct, G is the answer.