This is a transcription error.
Transcription errors can be classified according to many factors, e.g., location, type, etc. Therefore it may be relevant to your analysis to recognize that you have two clues: the error occurs at the base (versus the end or another location), and the error is a transcription between A and C.
A point mutation is a kind of transcription error, where a single nucleotide is replaced by another. However, you are not (yet) given complete information -- the statement does not preclude an additional insertion or deletion. In fact, my first read would be that this is what the researcher noticed -- a single change. Noticing one change often leads to further investigation. So, if you do make the assumption that this is a point mutation, you should also state the assumption that there are no other errors in this codon.
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