The science dealing with human variations and heredity leading to the improvement of the human race is known as genetics. Genetics is the branch of biology that studies the inheritance and variation of traits in living organisms, including humans. It focuses on how traits are passed down from one generation to the next and how they can be influenced by genetic factors.

Eugenics, which is mentioned in option 1, is a controversial belief system that aims to improve the genetic qualities of the human population through selective breeding and sterilization. It is not a science in itself but rather a social and political ideology.

Hybridization, mentioned in option 3, refers to the breeding of individuals from different species or subspecies to create offspring with desirable traits. While this process may be used in some scientific research and agricultural practices, it is not specifically related to the improvement of the human race.

Mutation, mentioned in option 4, refers to a change in the DNA sequence of a gene. While mutations can occur naturally and play a role in evolution, they are not directly related to the science of human variations and heredity leading to the improvement of the human race.

Therefore, the correct answer is option 2, genetics.