Which type of mutation adds one or more base pairs? insertion substitution deletion

Insertion.

An insertion is a type of DNA mutation where the addition of one or more nucleotide base pairs takes place in the DNA sequence. Insertions can be of many sizes and importance - they can be from only one base pair to a whole section of a chromosome, and they can be of more importance or less importance, for example, mutate a whole gene by insertion of a base pair in its sequence or to have multiple copies of a gene by the insertion of copies of that gene in a chromosome sequence.