Correct answer - Point mutation.
Why? - A point mutation or substitution is a genetic mutation where a SINGLE nucleotide base is changed, inserted or deleted from a DNA or RNA sequence of an organism's genome.
Why not Insertion - In genetics, an insertion (also called an insertion mutation) is the addition of one or more nucleotide base pairs into a DNA sequence. This can often happen in microsatellite regions due to the DNA polymerase slipping. ... On a chromosome level, an insertion refers to the insertion of a larger sequence into a chromosome. So, in short Insertion adds one nucleotide but not changes one nucleotide to another. Which is done by POINT MUTATION.
