What causes mutations to occur?

Mutations are changes in the genetic material that can occur due to a variety of factors, with one significant cause being errors that happen during DNA replication or as a result of damage. DNA replication is a critical process where the genetic code is duplicated to pass on to daughter cells. Occasionally, the machinery involved in this process can make mistakes, leading to base pair substitutions, insertions, or deletions, which are all forms of mutations.

In addition to replication errors, DNA can also be damaged by environmental factors such as radiation, chemicals, and biological agents, which can alter the structure of DNA. This damage can lead to a mutation if not repaired effectively by the cell's repair mechanisms. Both spontaneous errors and induced damage play essential roles in the mutational landscape of organisms, contributing to genetic variation that can drive evolution.

Therefore, choice C accurately captures the primary reasons behind mutation occurrence, highlighting the natural processes that lead to changes in the genetic code.