Two genes for the same trait are compared. which one is not an example of a mutation?
in the first, the series is adenine followed by two thymine; in the second, adenine is followed by two thymine.
in the first, the series is cytosine - adenine - thymine; in the second, it is cytosine - adenine - adenine.
the first has thymine where the second has a guanine.
the first has a series of three nucleotide bases; the other has a series of four.

The cytosine-adenine-adenine pair is not an example of mutation.

Explanation:

In the base pairing of DNA, adenine bonds only with thymine and guanine bonds with cytosine. As guanines and adenines are purines and thymine and cytosine are pyrimidine in DNA, and due to their structure, there is only small space in between them to fit a purine and pyrimidine and not more than that.

Therefore, these constructions of two adenines are not possible with cytosine. Thus, this cannot be an example of mutation.