Answer:
Cluster Sample
Step-by-step explanation:
According to ThoughtCo., "A cluster sample involves using a simple random sample of evident groups that the population contains." Since only the first students that enter the cafeteria are surveyed, not all grade levels are included. Since most schools do lunch by grade, only a certain grade would get to have a say, so there could be some bias.
(Edit: Looking back on this it might could be a convenience sample but it depends on what the answer choices are)