Final answer:
The landmark legislation that guaranteed education for students with disabilities in 1975 is the Education for All Handicapped Children Act, known as Individuals with Disabilities Education Act (IDEA) after 1990.
Step-by-step explanation:
The landmark piece of legislation that guaranteed a "free appropriate public education" for students with disabilities in 1975 is the Education for All Handicapped Children Act, which would later be renamed the Individuals with Disabilities Education Act (IDEA) in 1990. The act ensured that children with disabilities would be mainstreamed with all students, unless their specific disability prevented them from reaching their educational potential in that setting. IDEA has played a crucial role in increasing the quality of education for students with disabilities and has required schools to create individual educational plans that provide the maximal support needed for their best learning potential.