A base substitution mutation in a gene sometimes does not result in a different protein.?

C is correct. Three bases/codon and four types of bases. Four to the third power is 64 codons for twenty amino acids(AA). Subtract three as stops and you will see 61 codons for 20 AA. Watson or Crick explained this and called it wobble. The first two bases normally remain the same and the third varies. A couple AA like Leucine has 6 codons and the first base also is varied in that case.