Final answer:
The primary cause of the U.S. prison boom is attributed to the War on drugs, which includes strict "get tough on crime" policies and racial disparities in incarceration rates.
Step-by-step explanation:
The single most important cause of the prison boom in the United States is widely considered to be A. War on drugs. Starting in the 1980s, especially during the Reagan administration, there was a substantial increase in the number of people incarcerated as a result of the war on drugs. This was influenced by a political climate that emphasized "get tough on crime" policies, which included the introduction of more stringent laws such as mandatory minimum sentencing and "three strikes" legislation. The incarceration rates soared following these policies, and the prison system saw an influx of inmates convicted for drug-related offenses. Additionally, the racial disparities apparent in incarceration rates were significant, with minority groups being disproportionately affected. Economic incentives in the private prison industry, the election of judges and law enforcement leaders focused on toughness on crime, and changes in demographics also played significant roles in the rise of the prison population.