In the context provided, the completion of the sentence refers to a moment involving Harry walking through the Black fire. This scenario is reminiscent of the Triwizard Tournament's Third Task in "Harry Potter and the Goblet of Fire." During this event, contestants face various magical obstacles to reach the Triwizard Cup. The Black fire represents one such obstacle, and Harry successfully traverses it after casting a protective spell.
In the story, Harry is armed with the knowledge that he needs to protect himself from the dangers in the maze. To navigate through the Black fire safely, he uses a spell to create a magical barrier or shield around himself. This protective enchantment shields him from the harmful effects of the Black fire, allowing him to proceed towards his goal unharmed.
The other options, such as drinking a potion, mastering Occlumency, or rescuing Sirius Black, are not directly associated with the specific task of walking through the Black fire during the Triwizard Tournament. Therefore, based on the context provided, the most appropriate and accurate completion for the sentence is that Harry was able to walk through the Black fire after he cast a protective spell.