Answer:
Insertion.
Step-by-step explanation:
Insertion mutation:
In insertion mutation, the sequence of DNA permanently change by addition of one or more nucleotides. If mutiple of three nucleotides inserted into DNA sequence then there will be non-frameshift mutation occurs but if multiple of one or two nucleotides inserted into DNA sequence then it will be frameshift mutation.
Above scenario is a classical example of frameshift mutation by addition of one nucleotide.
The actual sequence is: The big red fly had one eye.
Now the addition of one nucleotide which shown in bold letter.
(wild type): The big red fly had one eye.
(mutant): The fbi gre dfl yha don eey.
So, insertion of one nucleotide (f) can lead to frameshift mutation.