Identify the Scenario Which Qualifies as a 'Common Mistake

Identify the scenario which qualifies as a 'common mistake'.

A) A beneficiary and an insurance company negotiated a life insurance policy,both unaware that the subject of the life insurance policy was in fact already dead.
B) X,who spoke little English,signed a document granting Y an option,believing it to be a receipt.This belief was encouraged by Y.
C) X purchased goods on credit from Y,believing Y to be Z.Y was aware of X's mistake.
D) Two parties agreed to send goods by a particular ship but there were two ships with that name and each party was thinking of a different ship.