Final answer:
Buddhist monks enter retreat during Vassa, the rainy season, for three months of spiritual practices. The Indian ruler who converted to Buddhism and spread its teachings was Ashoka.
Step-by-step explanation:
Buddhist monks typically enter into retreat during the Vassa period, which corresponds to the rainy season in India and other parts of Southeast Asia. This retreat lasts for three months, typically from July to October. It is a time for focused spiritual growth, meditation, and study. The other options, such as Vesak, Kathina, and Magha, represent different periods and festivals in the Buddhist calendar. Vesak, for example, celebrates the birth, enlightenment, and death of Buddha, whereas Kathina is the time when the laity offers new robes and other necessities to the monastic community following the end of Vassa.
The Indian ruler who promoted the spread of Buddhism after his own conversion was Ashoka. Ashoka was a significant figure in the history of Buddhism as he helped to propagate its teachings throughout the Indian subcontinent and beyond, after he embraced the religion following the bloody Kalinga war that led him to seek a path of peace.