The correct answer is because he had described Aenea’s journey to the underworld.
One reason Virgil is qualified to guide Dante on his descent through hell is that he had described Aenea’s journey to the underworld.
In the novel, Dante chooses Virgil as his guide to the underworld because he knew Dante was a respected writer and hoped people would consider his words wisely. The Roman Poet Virgil had finished the Eneid(Aeneid) by 19 BCE. The Inferno in Dante’s story is like an extension of the Aeneid in Virgil’s book six of the same name. So one reason Virgil is qualified to guide Dante on his descent through hell is that he had described Aenea’s journey to the underworld.