Final answer:
The name 'John' is the correct completion for the sentence based on the provided text excerpts which indicate a character by that name is relevant to the scenario described.
Step-by-step explanation:
The correct answer to the sentence completion question is: I was sitting outside the Commodore's mansion, waiting for my brother John to come out with news of the job. This answer can be determined from the provided excerpt that describes a character named John involved in the narrative, fitting the context of the question. Since the question is about a specific name, and the reference material repeatedly mentions a character by the name of John, we can confidently select John as the answer.
The name Charlie, James, and William, while common, do not find a direct reference in the material presented. Our knowledge of the character's involvement in activities outside the mansion and interactions with other characters reinforce this answer.