A is incorrect: The 'length' reason is given when the model reaches its token limit before completing the response, not when content is filtered for policy violations.

B is correct: The 'content_filter' finish reason is specifically returned to indicate that the output was intentionally withheld due to a violation of safety policies or content guidelines.

C is incorrect: The 'stop' reason indicates that the model completed its generation normally, not due to a content filter.

D is incorrect: A 'null' finish reason is atypical for a blocked response; a specific reason is usually provided.