Why Might a Point Mutation in DNA Make a Difference

Why might a point mutation in DNA make a difference in the level of a protein's activity?

A) It might result in a chromosomal translocation.
B) It might exchange one stop codon for another stop codon.
C) It might exchange one serine codon for a different serine codon.
D) It might substitute a different amino acid in the active site.
E) It might substitute the N-terminus of the polypeptide for the C-terminus.