The newspaper headline which sounds the least credible is C. New Stadium Is So Cool.
This headline could be the title of a social media post, such as a blog post, but it cannot be the headline of a newspaper because it sounds really informal. So, it would be more difficult for someone to believe that this could be a newspaper headline. The rest of the choices are all of the same level of formality and so they would be acceptable as possible headlines of a newspaper.
