The best sample for estimating the number of students who bring their lunch to school every day would be 80 students selected at random during lunch.
This option is most representative of the entire student body in terms of lunch habits because it captures a wide range of students at a time when they are actually making the decision to bring lunch or buy it. The other options are less ideal because they are narrower in scope (e.g., specific gender, academic year, or club membership), which might not accurately reflect the behaviors of all students.