Correct Answer: "is a mutation"
This is the best answer choice because a mutation when at least one base pair changes to another.
Incorrect Answers:
"would be a mutation only if it falls in a protein-coding part of a gene"
Some mutations are more influential than others. If this mutation occurred in a region that codes for a protein, then it could greatly influence the structure/function of the the protein and therefore the cellular processes that the protein is used for. Oppositely, mutations can occur in regions that don't code directly for a protein and therefore could go completely unnoticed, making this a bad answer choice.
"would be a mutation only if it falls in a transcribed part of the genome."
A mutation can occur when the gene in transcribed from DNA → RNA, but the process of transcription is separate from that of DNA replication so that makes this answer choice not so good.
"is not a mutation, because only one base pair has been altered."
A mutation can be just when one base pair is altered, so this is an incorrect statement.
