Respuesta :
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!!
The answer would be A.) you trade an apple for a banana at school. Because the barter system is when you trade something for something else.