The correct answer is option 1: Buddhism. Nalanda University, located in present-day Bihar, India, was indeed a renowned center of learning during ancient times. It flourished from the 5th century to the 12th century and attracted scholars and students from all over the world. While Nalanda did not exclusively focus on Buddhism, it was primarily known for its association with Buddhism. The university had a strong influence on the spread and development of Buddhist philosophy and teachings, making it a significant hub for Buddhist learning. Nalanda housed many Buddhist monasteries and temples and offered a wide range of subjects, including Buddhist studies, philosophy, logic, medicine, mathematics, and astronomy. Its libraries were renowned for their extensive collection of Buddhist scriptures and manuscripts.