The correct answer is: C. Kanishka
The 3rd Buddhist council was held in Kashmir in the 1st century CE, during the reign of the Kushan emperor Kanishka. The council was convened to resolve doctrinal disputes and to codify the Buddhist scriptures. It is also believed that the council was responsible for the introduction of Mahayana Buddhism to Central Asia.
Ajatashatru was the king of Magadha who defeated his father, Bimbisara, and became the ruler of the kingdom. He is not known to have been a patron of Buddhism.
Ashoka was the Mauryan emperor who converted to Buddhism after the Kalinga War. He is known for his patronage of Buddhism and for his efforts to spread the religion throughout his empire. However, he is not known to have been involved in the 3rd Buddhist council.
Harsha was the emperor of the Pushyabhuti dynasty who ruled over northern India in the 7th century CE. He is known for his patronage of Buddhism and for his construction of Buddhist monasteries and stupas. However, he is not known to have been involved in the 3rd Buddhist council.