Which of the Following Is the Best Description of 'Unilateral

Which of the following is the best description of 'unilateral mistake'?

A) Both parties are mistaken about a material fact that is important to the subject matter of the contract.
B) One of the parties is mistaken about a material fact regarding the subject matter of the contract, and the other party takes advantage of this mistake.
C) One of the parties is mistaken as to the nature of the document which they have signed.
D) Both parties have made the same mistake about the existence or identity of the subject matter of the contract.