To What Does the "Individual Mandate" in the Affordable Care

To what does the "individual mandate" in the Affordable Care Act refer?

A) the requirement that state governments always protect an individual's "right to die"
B) the requirement that state governments provide the same health care coverage to citizens and undocumented immigrants
C) the requirement that state governments expand their Medicaid coverage or lose all of their existing Medicaid funds from the federal government
D) the requirement that individuals purchase health care insurance coverage or pay a penalty to the federal government