Final answer:
Eugenics became popular at the end of the 1900s due to the belief in selective breeding and pseudo-scientific research. However, it fell into disrepute after Nazi Germany's eugenics program.
Step-by-step explanation:
Eugenics became prominent at the end of the 1900s due to a variety of ideological and scientific reasons. One reason was the prevailing belief in the concept of selective breeding to improve society. This belief was supported by leading organizations and scientists who conducted pseudo-scientific research to validate their conclusions about the supposed negative genetic traits of certain groups. Additionally, racism, xenophobia, and the fear of degeneration fueled the popularity of eugenics. However, the eugenics movement fell into disrepute after Nazi Germany implemented an extensive eugenics program in the 1930s and 40s.