Final answer:
The Affordable Care Act (ACA) saw the most significant increase in health insurance enrollment through state-sponsored health exchange markets and Medicaid, and a decrease in the uninsured or underinsured population.
Step-by-step explanation:
According to the information provided, the largest increase in enrollment in health insurance because of the Affordable Care Act (ACA) is expected to occur among those signing up through state government sponsored health exchange markets or Medicaid, while the largest decrease is expected to be among the uninsured or underinsured. This is a reflection of the ACA's success in reducing the number of Americans without health insurance from 20.3% in 2012 to 11.5% in 2016, and eventually to 8.6% as of 2020. The ACA, sometimes referred to as Obamacare, mandated that individuals with preexisting conditions could no longer be denied health insurance and required Americans to buy health insurance, thus substantially increasing coverage.