Population: The entire number of students present in town's high school.
Sample: Identical number of students across various categories of fresher's, senior students & juniors.
Population is the large set of the people who are present during the experiment. Sample is a smaller subset from the experiment. In this question, samples are the students who are either new joiners to the college, the senior students or the juniors. It will help Alan to come to a possible conclusion using the survey.