The correct answer is: B. Asoka
The First Buddhist Council was held in Rajgriha under the patronage of Emperor Asoka the Great. It was convened about 100 years after the death of Gautama Buddha, in order to resolve disputes that had arisen among the Buddhist sangha (monastic community). The council also standardized the Buddhist canon and established the fourfold division of Buddhist monks into bhikkhus, bhikkhunis, upasakas, and upasikas.
Chandragupta Maurya was the founder of the Mauryan Empire, which ruled over most of the Indian subcontinent from the 4th to the 2nd centuries BCE. He was not a Buddhist, and there is no evidence that he patronized the Buddhist sangha.
Ajatashatru was the king of Magadha who reigned from 493 to 462 BCE. He was the son of King Bimbisara and the father of Emperor Ashoka. He was a patron of the Buddhist sangha, and he is said to have built the Mahabodhi Temple in Bodh Gaya, the site of the Buddha’s enlightenment.
Kanishka was the Kushan emperor who reigned from 127 to 150 CE. He was a patron of Buddhism, and he is said to have convened the Fourth Buddhist Council in Kashmir.
Therefore, the correct answer is: B. Asoka