C. a proposal from a local radio station detailing a popular ad
Because those holding the bake sale would know how much they are charging sponsors for sponsorships and would also know the number of people who would come into contact with the sponsor information, the information that would be needed is the same information for a radio ad campaign in order to make the comparison. As such a proposal from a local radio station detailing a popular ad plan would be just the source needed.
????? What’s the question
Well the best answer I can give would be if it doesn't match all the other websites that are like it and their answers don't line up than its not reliable but if you can find that all the websites are linked in some way than they should be reliable.