What is a mutagen?

A mutagen is defined as a substance or factor that increases the rate of changes to the DNA sequence. This could involve various agents such as chemicals, radiation, or biological processes that can alter the DNA sequence, either by causing mutations directly or by failing to repair DNA damage effectively. The understanding of mutagens is crucial in genetics and molecular biology because they can lead to significant biological consequences, such as the development of cancer or other genetic disorders.

The correct choice highlights that mutagens have a direct impact on the genetic material, resulting in heritable changes that can affect an organism's traits, development, and health. This aligns with our understanding of how mutations can arise in nature and be induced by external factors.