Eugenics is the study of

The correct answer is D. Eugenics is the study of altering human beings by changing their genetic components.

Eugenics is a set of beliefs and practices that aim to improve the genetic composition of a population. This can be done through selective breeding, genetic engineering, or other methods. Eugenics has been used to justify a variety of harmful practices, including forced sterilization and genocide.

Option A is incorrect because eugenics is not the study of the genetics of plants. Option B is incorrect because eugenics is not the study of different races of mankind. Option C is incorrect because eugenics is not the study of people of European origin.