Choose the grammatical error in the sentence: "The car that he had seen at the Ford dealer was the one that he final decided to buy."

The grammatical error in the sentence is the use of "final" instead of "finally." The word "final" is an adjective that describes something as being last or concluding, while "finally" is an adverb used to indicate that something occurs at the end or after a long time. In this context, the intent is to convey that the decision to buy the car came after consideration, which is appropriately expressed with the adverb "finally." Thus, replacing "final" with "finally" would correct the sentence, allowing it to convey the intended meaning that he came to this decision at the conclusion of his thoughts or reflections about the purchase.