Final answer:
The Health Maintenance Organization Assistance Act of 1973 required most employers with more than 25 employees to offer HMO coverage if local plans were available, which is separate from the Affordable Care Act, also known as Obamacare, enacted in 2010.
Step-by-step explanation:
The legislation that required most employers with more than 25 employees to offer HMO coverage if local plans were available is the Health Maintenance Organization Assistance Act of 1973. The Patient Protection and Affordable Care Act, more commonly known as Obamacare and signed into law in March of 2010 by President Obama, represented a significant overhaul of the U.S. healthcare system, aiming to provide universal coverage and including the requirement for U.S. citizens to have some form of health insurance. While the Affordable Care Act has been a significant piece of legislation for healthcare reform, the specific requirement for employers to offer HMO coverage predates the ACA and is instead associated with the 1973 Act.