<span>neutral stimulus is the correct answer </span>
Answer:
water, animals, shelter (plants; trees, rocks)
Explanation:
Barter system- A method of exchange when no money is involved.
B and C can't be the answers because they are not exchanging anything.
D can't be the answer either because there is money involved.
So your answer is A) You trade an apple for a banana at school. because you are exchanging something with someone for something else thats not money.
Hope this Helps!!