The provision of the Civil Rights Act of 1964 that invalidated the “separate but equal” doctrine established by Plessy v. Ferguson is Title II: Public Accommodations.
Title II prohibits discrimination on the basis of race, color, religion, or national origin in places of public accommodation, which includes hotels, restaurants, theaters, and other establishments. This provision was instrumental in challenging and dismantling the legal basis for segregated public facilities that were justified under the "separate but equal" doctrine. By affirming that discrimination in public accommodations is unlawful, Title II effectively invalidated the rationale that supported racial segregation upheld in Plessy v. Ferguson.