Final answer:
The Cruciatus Curse is the Unforgivable Curse used to torture a victim, causing excruciating pain. It has long-term damaging effects, differentiating it from the other Unforgivable Curses and the Stupefy spell.
Step-by-step explanation:
The Unforgivable Curses are three of the most powerful and sinister spells in the wizarding world, used to inflict harm, control, or even kill the victim. The curse used specifically to torture the victim is the B) Cruciatus Curse. When cast effectively, it inflicts excruciating pain upon the recipient, equivalent to having hot knives driven into the victim's nerves. This curse is known for causing severe physical and psychological damage, and prolonged exposure can lead to serious long-term effects, including permanent mental incapacitation. The other options listed are also Unforgivable Curses with different effects: A) Imperius Curse controls the victim's actions, C) Killing Curse (Avada Kedavra) causes instantaneous death without any marks or suffering, and D) Stupefy is not an Unforgivable Curse but rather a stunning spell used to render a target unconscious.