A ruling of the FTC requires that every used car must display a(n) ________ spelling out the warranty terms under which the vehicle is offered for sale.

To arrive at the answer, you can follow these steps:

1. Understand the question: The question asks for the term used to describe what must be displayed on a used car to outline the warranty terms.
2. Identify the context: The question mentions the FTC (Federal Trade Commission), which suggests a legal or regulatory context.
3. Break down the answer choices:
A. Vehicle identification chart: This seems unrelated to warranty terms.
B. Agreement: While a used car sale may involve an agreement, it may not specifically pertain to the warranty terms.
C. Option chart: An option chart is unlikely to be related to warranty terms.
D. Buyer's guide: This seems the most relevant option since it can outline the warranty terms for a used car.
4. Determine the correct answer: Based on the analysis, the most appropriate answer is D. buyer's guide.

Therefore, the correct answer is D. buyer's guide.