Answer:
3rd N-base to go from UCU to UUU
Explanation:
a) substitution of 3rd N-base to go from CAU to CAC will cause a silent mutation which means that the change in nucleotide sequence will not change the encoding aminoacid  i.e Histidine
b) substitution of 3rd N-base to go from UGU to UGC will also results in silent mutation which means that the change in nucleotide from U to C here will not change the encoding aminoacid  i.e Cysteine
c) Â the substitution of 3rd N-base to go from UCU to UUU has the potential to cause the damage because in this option the nucleotide change cause the change in encoding amino acid sequence i.e from Serine to Phenylalanine, so it can affect the protein function and stability further. This type of mutation is called as missense mutation.