Mutagens are substances that can affect what aspect of an organism?

Mutagens are substances that cause changes or alterations in the genetic material of an organism, specifically DNA. They can lead to mutations, which can have various effects on an organism's genetic system. This includes possible changes to how the organism develops, functions, or reproduces. By affecting the genetic code, mutagens can influence traits that can be passed on to future generations, meaning their impact is significant on evolution and heredity.

The other aspects mentioned in the choices, such as physical characteristics, immune response, and metabolism, can be influenced by genetic changes that mutagens cause. However, the definitive action of a mutagen is its direct impact on the genetic system itself, which makes it the correct response in this context.