Gene Mutations Consist of Changes in a Single Gene and Can

Gene mutations consist of changes in a single gene and can be base substitutions (a single pair of nucleotides is altered) or insertions or deletions (nucleotides are added or removed) . A base substitution can be a transition (substitution of like bases) or a transversion (substitution of unlike bases) . Insertions and deletions often lead to a change in the reading frame of a gene.
-Which of the following changes is a transition base substitution?

A) Adenine is replaced by thymine.
B) Cytosine is replaced by adenine.
C) Guanine is replaced by adenine.
D) Three nucleotide pairs are inserted into DNA.