What type of mutation occurs when DNA changes from GGTTAG to GGATAG?

The correct choice is nucleotide base substitution because the DNA sequence changes at a specific position without altering the overall length of the sequence. In this instance, the second 'T' in the original sequence (GGTTAG) is replaced with an 'A', resulting in the new sequence (GGATAG). This type of mutation specifically involves replacing one nucleotide with another, which can occur due to various factors such as errors during DNA replication or external influences like radiation.

In this scenario, the original sequence and the mutated sequence maintain the same number of nucleotides (6 in total), emphasizing that the structure of the DNA strand remains intact aside from the substitution of one base. This type of mutation can lead to different outcomes depending on the specific role of the substituted nucleotide, potentially affecting protein coding sequences or regulatory regions.

Contextually, other options describe different forms of genetic changes: insertion would add extra nucleotides, deletion would remove them, and chromosomal rearrangement refers to larger-scale changes in chromosome structure. However, none of these apply in this case, as the change involves a direct and singular alteration in the sequence.