The correct answer is Nagasena.

Nagasena was a renowned Buddhist sage who played a significant role in the spread of Buddhism during the reign of the Indo-Greek king Menander I (Milinda). Historical accounts, particularly the *Milindapañha* (Questions of Milinda), detail a philosophical discourse between Nagasena and King Menander. This text showcases Nagasena's profound understanding of Buddhist principles and his ability to engage in intellectual debate with a learned king. The *Milindapañha* doesn't merely present a historical record but also serves as a valuable source for understanding Abhidhamma, a significant school of Buddhist thought. Through this dialogue, Nagasena effectively conveyed the essence of Buddhism, leading to Menander's conversion. Other significant Buddhist figures mentioned might have contributed to the spread of Buddhism during that period but it's specifically Nagasena's interaction with Menander that is well documented and forms the basis of the answer. The depth of the philosophical discussions recorded emphasizes Nagasena's impact on Menander's understanding and subsequent adoption of Buddhism. The historical significance of this event remains crucial in understanding the intersection of Buddhism and Greco-Buddhist culture in ancient India.