Which statement summarizes the content or main idea of this contract?

This is the only answer that includes all the main idea elements.
Answer "A buyer and seller are still discussing the price of the car" is incorrect because you sign a contract only after the details (including the price) have been decided, while answer "The buyer will fill out the contract because all the responsibility is with him or her" is an incorrect detail.
Answer "The buyer and seller agree to terms of the sale and promise to abide by them" is correct as far as it goes, but it is not the complete main idea.