Final answer:
The ad hominem response to the statement would be option B - 'You're only saying that because you're an atheist.'
Step-by-step explanation:
The ad hominem response to the statement, 'We shouldn't allow prayer in schools because of the separation of church and state,' would be option B - 'You're only saying that because you're an atheist.'
Ad hominem is a fallacy where instead of addressing the argument, the response attacks the person making the argument. In this case, the response is attacking the person's belief system rather than addressing the separation of church and state argument. This response is invalid because it does not provide a valid counter-argument to the original statement.