If you wanted to find the mean height of students in an elementary school,

which group would provide the most accurate sample?

It is not possible to select a single group from the primary school, since being a single group it could not in any way represent the entire primary school.

Choosing a group would provide the average height for that group, not the entire primary.

Assumptions can be made that the course of half of the primary would theoretically give us a more accurate representation, but the objective is to select a number of students from each course and there would be a representative sample of the entire primary.