If this code is in your header, I’m guessing you’re trying to pull this field in from an options page?
If that’s the case, you just need to add in another parameter to get_field().
So it would become get_field('options_posts','option').
If you’re just trying to pull in the field from the page you’re on, I’m assuming your header is being called before the WP loop starts. In that case, you’ll need to pass the page/post ID to get_field() instead. Let me know if you need more clarification.