Answer: c. preventing a market that would generate mutually beneficial trades.
Explanation:
Zooey could argue that the policy of lunch trades is preventing a market that would generate mutually beneficial trades because if people were allowed to trade what they want for what they have with people who have what the first person wants and wants what the first person has, that can be beneficial to both of them.
It is not unlike the system of batter trading that existed before money where people traded what they had for what they wanted.
One should be very careful here though because there are multiple disadvantages involved such as kids exchanging away more nutritious food and food poisoning.