The correct answer is: B. Ashoka
Buddhism was introduced to Nepal during the reign of Ashoka the Great, who ruled the Mauryan Empire from 272 to 232 BCE. Ashoka was a devout Buddhist and he spread the religion throughout his empire, including Nepal. He built many Buddhist temples and stupas in Nepal, and he also sent Buddhist missionaries to the country.
Samudragupta was a Gupta emperor who ruled from 335 to 375 CE. He was a powerful ruler who expanded the Gupta Empire to its greatest extent. However, there is no evidence that he introduced Buddhism to Nepal.
Chandragupta Maurya was the founder of the Mauryan Empire. He ruled from 321 to 297 BCE. He was a powerful ruler who united much of the Indian subcontinent under his rule. However, there is no evidence that he introduced Buddhism to Nepal.
Harshavardhana was an Indian emperor who ruled from 606 to 647 CE. He was a powerful ruler who expanded the Harsha Empire to its greatest extent. However, there is no evidence that he introduced Buddhism to Nepal.
In conclusion, Buddhism was introduced to Nepal during the reign of Ashoka the Great.