Choice "C" is correct. The parol evidence rule generally bars evidence of prior or contemporaneous oral statements offered to vary the terms of a fully integrated written contract. Oral evidence is permissible when the contract is incomplete, ambiguous, invalid, or subject to a condition precedent, or when modification is made after the original contract is written. A contemporaneous oral agreement will be excluded.
Choice "d" is incorrect. Oral evidence is admissible to prove the failure of a condition precedent since this does not vary the terms of the contract but rather is a collateral issue. The parol evidence rule bars only prior or contemporaneous oral statements that seek to vary the contract's terms.
Choice "b" is incorrect. The parol evidence rule bars only prior and contemporaneous oral statements that seek to vary the terms of a contract. The lack of contractual capacity is not relevant to the parol evidence rule.
Choice "a" is incorrect. The parol evidence rule bars evidence of prior or contemporaneous statements that contradict a written contract. It does not exclude evidence of subsequent statements.