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, such as hotels, restaurants, and theaters. This marked a significant step in dismantling segregation and challenging the legal foundation of the separate but equal doctrine, which had allowed for racial segregation under the premise that it was constitutionally permissible as long as facilities were equal. By ensuring that everyone had equal access to public accommodations, Title II effectively challenged and undermined that doctrine.
While other titles of the Civil Rights Act address related issues, Title II is the one most directly associated with the invalidation of the separate but equal ruling in public spaces.