Answer:
D, u selected the right one
Explanation:
A: asking the teachers who have cafeteria duty won't help you find out why students don't want to buy lunch.
B: the students that are already waiting in line for lunch won't help you find out why student don't want to buy lunch either because they are buying the lunch.
C: Asking all the teachers won't help because you want to know why students don't buy lunch
D: Asking students in the school will help because the students will be able to tell you exactly why THEY don't buy lunch.