Which is the greatest contribution of Gregor Mendel to society?

an ability to beautify the environment

the variety in breeds of dogs and cats

the increased speed by running horses

an understanding of genetic principles

I would say D. "An understanding of genetic principles" because, by matter of elimination, out of the choices (or answers) given, His understanding for genetic principles most changed outlook of The science of inheritance and genetics, and it lead to major progress to where we are today.