Answer:
Sampling frame
Explanation:
A sampling frame is the source where the researcher draws his samples from. In other words, a sampling frame is like the population where the researcher gets samples for his investigation. In the above example, we are told the researcher wants to know why the 72% that subscribed last year didn't subscribe this year. He draws a sample of 8% from the 72% that he is trying to study, therefore the 72% are the sampling frame from where he gets his sample.