By the end of early adulthood, the most common reason for death is

The correct answer is disease.
As we grow older, we are more likely to contract a particular disease, often with lethal results. The rate of suicide, homicide, and automobile accidents is not as high as in earlier years, but the risk of getting a disease and dying from it progressively rises as we grow older and older. So disease is the most common reason for death, usually heart attack or cancer.