If a base-pair change occurs during dna replication, this

is a mutation.
would be a mutation only if it falls in a protein-coding part of a gene.
would be a mutation only if it falls in a transcribed part of the genome.
is not a mutation, because only one base pair has been altered.

Correct "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.