Why haven't they allowed you to go?
Answer: B
The present perfect form ‘have/haven’t + verb’ in active voice gets converted into ‘haven’t been + verb’ in passive voice. Also, an indefinite or vague pronoun (such as they, we) which is used in the active voice is avoided in passive. There is no need to put the agent with by. Keeping these conditions in mind, the most appropriate answer is option (b).