Final answer:
The parol evidence rule preserves the sanctity of the written agreement by prohibiting the introduction of extrinsic evidence to vary or contradict the terms of a written contract.
Step-by-step explanation:
The parol evidence rule is a rule of contract law that prohibits the introduction of extrinsic evidence (evidence outside the written agreement) to vary or contradict the terms of a written contract that the parties intended to be a complete and final expression of their agreement.
The purpose of this rule is to preserve the sanctity of the written agreement by ensuring that the terms of the written contract are given effect as the final expression of the parties' intentions. Therefore, the correct statement describing the effect of the parol evidence rule is option A: The rule preserves the sanctity of the written agreement.