If Anna by Mistake Delivers to Bob a Plain, Unaddressed

Multiple Choice

If Anna by mistake delivers to Bob a plain, unaddressed envelope containing $50 intended for Cora:

A) Bob is under no obligation to return the money.
B) Bob is under a contractual obligation to return the money.
C) Bob's obligation to return the money is quasi-contractual.
D) Bob's obligation to return the money is implied in fact.