Which of the following is not a type of mutation?

A duplication mutation is indeed considered a valid form of mutation. It involves the duplication of a segment of DNA, leading to the repetition of genes or other genetic elements. This type of mutation can arise from errors during DNA replication or from unequal crossing over during meiosis.

In contrast, missense mutations, nonsense mutations, and silent mutations are all specific types of point mutations that alter individual nucleotides within a gene.

Missense mutations result in a different amino acid being incorporated into a protein, which can affect the protein's function. Nonsense mutations create a premature stop codon, leading to a truncated and usually nonfunctional protein. Silent mutations do not change the amino acid sequence of a protein due to the redundancy of the genetic code, often resulting in no effect on protein function. Each of these types contributes to genetic variability and can have various effects on organisms, demonstrating different aspects of how mutations can alter gene expression and protein synthesis.