Q: In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.
Question: How old was Charlemagne's son when he destroyed the Ring of the Avars? Passage:Since about 560, the Avars had ruled over large parts of the Pannonian Basin and Carantania southeast of Francia, though in the last decades, the Avar Khaganate had to deal with Bulgarian and Croat incursions. To secure the frontier of his empire and the traffic on the trade routes to the east, Charlemagne allied with Khan Krum of Bulgaria and the Croatian duke Vojnomir, and from 791 launched several campaigns against the Avars, where he, according to the Vita Karoli Magni, encountered only modest opposition. In 796 Carolingian forces under Charlemagne's son Pepin of Italy destroyed the main Avar fortress called the Ring of the Avars and made the Khagan a Frankish vassal, while the remaining Avars retreated behind the Tisza River. The victory is perpetuated by the poem De Pippini regis Victoria Avarica. 
 Links: a. Francia b. Khagan c. First Bulgarian Empire d. Pepin of Italy
A:
d