The provision that invalidated the "separate but equal" doctrine established by the Supreme Court's decision in 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 public facilities. This section effectively challenged and sought to dismantle the legal backing for segregation in public spaces, which was upheld by the "separate but equal" doctrine.