The best sample for estimating the number of students who bring their lunch to school every day is:
80 students selected at random during lunch.
This option is the most representative of the entire student body because it includes a larger number of students and captures a specific time when students are likely making choices about their lunch. The other options (members of the Math Club, first-year students, and female students) may not provide a representative sample of the entire student population, as they are limited to specific groups.