Eric wants to find out the most popular after-school activity among high school seniors at his school. what is the population in eric's survey

I believe the answer is: all high school seniors at Eric's school

In survey, population refers to the group of subjects that can be used as a source to collect the data for the survey.

From the description above, eric wants to find out the most popular after-school activity among high school seniors.

This means that all high school seniors' opinion could be used as the data for this question.