Answer: All listeners of the radio talk show
Explanation: In simple words, the part of the population that could be used for the research conducted is called the implied population of such research.
In the given case, all the listeners were the implied population as every one had the chance to participate. The callers are the sample of the population. Sample refers to the part of population actually used in the conduct of research.
Hence from the above we can conclude that the correct option is D.