Final answer:
mRNA mutations have a more significant effect on gene expression because they directly affect the production of proteins.
Step-by-step explanation:
The correct answer is: b) mRNA mutation
Mutations in both the DNA and mRNA can have an effect on gene expression, but mRNA mutations have a more significant effect. This is because the mRNA is directly involved in the translation process, which results in the production of proteins. Any mutation in the mRNA molecule can directly affect the sequence of the resulting protein, leading to the production of defective proteins.