The mrna codon cuu could mutate to and still code for leucine, which could be a neutral mutation. what is the blank?

A codon is a sequence of three bases found on the messenger RNA (mRNA) molecule which is translated to produce an amino acid. Each amino acid is coded by one or more specific codons. If the codon is changed through a mutation, the amino acid produced may be different. However, in this case, the other codons that produce Leucine are:

UUA, UUG, CUU, CUC, CUA and CUG

So the code CUU could mutate to CUC, CUA or CUG and leucine would still be produced.