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B is corrent because the parol evidence rule states that once an agreement is reduced to a writing intended as the complete contract, the parties may not introduce oral or written evidence in an attempt to alter or contradict the terms of the agreement. Thus, Elrod would not be able to introduce the evidence if the contract indicates that it was intended as the entire contract, and the disputed point is covered in detail in the agreement.
A is incorrect because the parol evidence rule applies only to evidence arising prior to the formation of the written contract; evidence which arises after the formation of the contract is admissible.
C is incorrect because the parol evidence rule does not bar from admission any evidence which will prove that a contract is invalid. Therefore, evidence in support of the contention that a mutual mistake is present would be admissable.
D is incorrect because the parol evidence rule does not bar evidence which will clarify an ambiguous point in the written contract.