The Buddhist sect Mahayana formally came into existence during the reign of

The correct answer is: D. Kanishka

Mahayana Buddhism is a major branch of Buddhism that originated in India sometime between the 1st and 5th centuries CE. It is the dominant form of Buddhism in East Asia, Southeast Asia, and Tibet. Mahayana Buddhism is characterized by its emphasis on the bodhisattva ideal, the goal of achieving enlightenment for the benefit of all beings. It also includes a number of different schools and traditions, each with its own unique teachings and practices.

The reign of Kanishka (c. 127-151 CE) is considered to be a pivotal period in the development of Mahayana Buddhism. Kanishka was a Kushan emperor who ruled over a vast empire that included much of Central Asia and northern India. He was a patron of Buddhism and is credited with convening the Fourth Buddhist Council, which is said to have been held in Kashmir. The council is said to have standardized the Mahayana scriptures and to have promoted the spread of Mahayana Buddhism throughout the empire.

The other options are incorrect because:

• Ajatashatru (c. 492-462 BCE) was the king of Magadha who is said to have killed his father, Bimbisara, in order to seize the throne. He is not known to have been a patron of Buddhism.
• Ashoka (c. 272-232 BCE) was the Mauryan emperor who is known for his Buddhist reforms. He is credited with converting to Buddhism after the Kalinga War and with promoting the spread of Buddhism throughout his empire. However, Mahayana Buddhism did not formally come into existence until after his death.
• Dharmapala (c. 770-810 CE) was the Pala emperor who ruled over Bengal and Bihar. He was a patron of Buddhism and is credited with restoring many Buddhist monasteries and temples. However, Mahayana Buddhism had already been in existence for centuries by his time.
• Kanishka (c. 127-151 CE) was the Kushan emperor who is considered to be a pivotal figure in the development of Mahayana Buddhism. He is credited with convening the Fourth Buddhist Council and with promoting the spread of Mahayana Buddhism throughout his empire.