The correct answer is "radio."
This broadcast was given in 1938, a year when millions of American relied on the radio for news and entertainment. At this time, TV's were still in their infant stages and were not mainstream by any means. With this in mind, you can see how radio would be the next best option.
So when Orson Welles did this broadcast, he knew that he had the possibility of millions of Americans listening. However, he did not anticipate that they would interpret the events as real.