Refer to Fig. 6.4.1 and 6.5.2 for this and the next two questions. The mutation occurs in the DNA, but questions refer to results in the mRNA. The original codon is UAU, which codes for the amino acid tyrosine (Tyr). (You might want to confirm this.) A mutation that changes the first letter to 'C' is called:

A mutation that changes the first letter to 'C' is called Frameshift mutation.

Explanation:

 FRAMESHIFT MUTATION -: A frameshift mutation is a genetic mutation of a number of nucleotides caused by indels (insertions or deletions) in a DNA sequence that is not divisible by three.  A frameshift mutation (also called a framing error or a reading frame shift) The addition or deletion may alter the reading frame (the grouping of codons) due to the triplet nature of gene expression by codons resulting in a completely different translation from the root. A frameshift mutation, instead of being added or removed, is not the same as a single-nucleotide polymorphism in which a nucleotide is substituted. In general, a frameshift mutation will cause the reading of the codons to code for various amino acids after the mutation. The first stop codon ("UAA", "UGA" or "UAG") encountered in the series would also change the frameshift mutation. The polypeptide that is formed will be abnormally short or abnormally long, and most likely will not be functional.Hence, the answer is Frameshift Mutation.