As a result of a mutation, the DNA sequence AGT - TGA - CAT changes to AAT - TGA - CAT.

Which best describes the mutation?

It is a substitution in which the DNA base, G, was replaced with a different DNA base, A.
It is a deletion in which the DNA base, G, was replaced with a different DNA base, A.
It is a substitution in which three DNA bases, AGT, were replaced with three different DNA bases, AAT.
It is a deletion in which three DNA bases, AGT, were replaced with three different DNA bases, AAT.