It is reasonable to use the entire population instead of a sample in the following situation:
A teacher wants to know his students’ favorite holiday in all of his math classes.
In this case, the teacher is likely to have access to all the students in his classes, making it feasible to gather data from the entire population of interest (the students in his math classes) rather than relying on a sample.
In contrast, the other situations involve larger populations (the trees in a national forest, customers of a store, and students in a university), for which sampling would typically be more practical and efficient.