The correct answer is C. A proposal from a local radio station detailing a popular ad plan
Explanation:
An assertion refers to a positive statement or declaration that usually describes the way some situation or issue is. However, for assertions to be valid an not just a personal opinion or point of view, the author of the assertion needs to provide evidence or information from external sources that prove the content of the assertion is a fact and therefore is valid. In general terms, a source supports effectively an assert when it provides evidence that explains or back up the information of the assertion.
This means in the case of the assert "The cost of sponsoring our bake sale is much less than a radio ad campaign and you will reach just as many people" the supporting source should prove the money spend on radio ad campaign is more than the money they already spend in sponsoring and therefore the most suitable source to support the assert this "A proposal from a local radio station detailing a popular ad plan". Because this allows the author to compare the prices of both ad campaigns and proving one is cheaper than the other..
