Final answer:
The Rehabilitation Act of 1973, particularly Section 504, enabled NIMH to reform mental health care provision by providing equal opportunity and accessibility for the mentally disabled.
Step-by-step explanation:
The law that enabled the National Institute of Mental Health (NIMH) to reform mental health care provision was the Rehabilitation Act of 1973. Specifically, Section 504 of this act provided equal opportunity for employment for federal government programs for both physically and mentally disabled persons and allowed for accessibility improvements. The act was part of broader social reforms during an era that saw increased government involvement in healthcare, education, and welfare under President Lyndon Johnson's administration and subsequent legislation.