In its landmark decision upholding the most significant of the health reform laws, the Supreme Court held which provision unconstitutional?

The mandatory expansion of state Medicaid programs

Explanation:

In 2012, the U. S Supreme Court decided to keep Obama's health reform law which established, as a requirement, that the majority of Americans buy their own health insurance starting in 2014, and declared that such reformation was not unconstitutional. Therefore, limiting a huge expansion of Medicaid, the health care program that provides for those in need, which back then was growing rapidly and being used by many Americans.