In this case, keeping the extra $10 would be considered as unethical but not illegal.
Ethical behavior is a matter of moral principles and values while legality is determined by the law. When the cashier makes a mistake and gives you more change than you are owed, you have a moral obligation to inform the cashier of the error and return the extra money.
Keeping the extra money when you know it's a mistake would be considered unethical because it goes against principles of honesty and fairness.
So, therefore, in this situation, keeping the extra $10 in change would be considered unethical but not illegal. The Option B is correct.
The complete question is:
If a cashier mistakenly gives you $20 back in change when he should have given you just $10, keeping the extra $10 would be:
A. illegal, but not unethical.
B. unethical, but not illegal.
C. irresponsible, but not unethical.
D. illegal and unethical.
E. None of the above