Answer:
We know that Alan is conducting a survey to find out the type of art preferred by students in the town's high school.
So, to drive a proper study, he need to take all students in the town's high school as population, because that's the objective.
However, when we drive a statistical study, in most of the cases, we aren't able to apply the study to all the people who represent the population. In that case, we take a population sample, which must represents the total number of students.
The sample is a sub-group of the population, and it must have the same characteristics and qualities as the population, otherwise, the study will be biased.
So, the sample a possible sample must be a certain number of students from each year. First year students, second year students, third year students, and so on. That way the sample will represent the population properly.