Ramakrishna taught Swami Vivekananda Advaita Vedanta philosophy with the key teaching that "service to man was the most effective worship of God" (Jiva is Shiva). This principle became the foundation of Vivekananda's later work and the Ramakrishna Mission's service-oriented approach. Options A, B, and D don't represent Ramakrishna's core teachings to Vivekananda.