The correct explanation for why the substitution of the nucleotide does not change the physical trait (fur color) in the mouse is:
The substituted codons code for the same amino acid as the original nucleotide codon.
This phenomenon is known as a "silent mutation" or "synonymous mutation," where a change in the nucleotide sequence does not result in a change in the amino acid sequence of the protein. Since the protein that governs fur color remains unchanged, the physical trait (fur color) also remains the same.