Answer: B. Betrayal of friends.
Explanation: according to Dante's Inferno, there are nine circles of Hell, being the first one for the most mild sins and then while he goes down in the circles, the sins are more serious, until the circle number nine, which is reserved to the worst sinners. The first circle is the limbo which was like an inferior form of heaven, and then we can find circles for sins like lust, gluttony, greed, anger, etc, but the last and worst circle is for people that commit treachery, so we can infer that Dante considered the betrayal of friends monstrous.