Final answer:
The term for the average number of people estimated to have listened to a radio station for at least five minutes during any quarter-hour, expressed as a percentage of the population, is the 'average quarter-hour rating'.
Step-by-step explanation:
The term you are looking for to describe the average number of people estimated to have listened to a radio station for a minimum of five minutes during any quarter-hour in a time period, expressed as a percentage of the survey area population, is the average quarter-hour rating (C). The average quarter-hour rating provides insight into a radio station's viewership by indicating what percentage of a given audience is tuned in at a specific time. Other related terms such as cumulative audience may refer to the total number of different listeners over a period, and average quarter-hour share refers to the percentage of the total listening audience captured by the station during a specific time. However, it is the average quarter-hour rating that matches the definition you provided.