What type of mutation involves the addition of an extra base to DNA?

The correct answer is the type of mutation known as an insertion, which refers to the addition of one or more nucleotide bases into a DNA sequence. This can lead to changes in the reading frame during the process of translation, potentially resulting in significant alterations to the protein that is produced. Insertions can occur due to errors in DNA replication or as a result of cellular processes such as transposon activity.

This type of mutation is particularly impactful because it can completely change the amino acid sequence downstream of the insertion point, leading to a different and possibly nonfunctional protein. Understanding insertions and their effects on genetic material is crucial for grasping how mutations can affect an organism's phenotype.