What does the term 'germ plasm' refer to?

The term 'germ plasm' specifically refers to the genetic material found within reproductive cells, such as eggs and sperm. This includes the DNA that is passed from generation to generation, containing the hereditary information necessary for the development and functioning of an organism. Germ plasm is crucial in the study of genetics and evolution, as it plays a vital role in the inheritance of traits and biological diversity within populations.

The other options focus on different concepts: physical traits and visible characteristics pertain to phenotypes, which are expressions of genetic information rather than the genetic material itself. Environmental factors relate to external influences that can affect genetic diversity but do not constitute germ plasm. Hence, recognizing germ plasm as the genetic material of reproductive cells is fundamental to understanding how traits are inherited and how evolution occurs.