The answer:
D boy
Explanation:
Boy A is sitting on a tree branch doing nothing → This is considered normal.
Boy B is cutting the branch where Boy A is sitting, not the one he’s sitting on himself → Still safe for himself.
Boy C is cutting the branch that Boy A and Boy B are sitting on → Boy C remains safe while cutting.
Boy D is cutting the very branch he’s sitting on → This is dangerous for himself, so he is considered the most foolish.
Question: Who is most stupid?
