This question is asking you to pick the answer choices which identifies the assumption buried in the argument. This means you are picking the choice that is necessary for the argument to be true. You can test your answer choice by negating it. If it is the correct answer, and it is negated, the whole argument should fall apart.
Answer (D) is correct because it is required for the argument. We are told that Clark parts are best because they satisfy our government standards. But, we don't know anything about these government standards, other than that they are the "toughest in the world." That still could mean all the standards out there, including our government's, are really poor standards. If that was the case, just having the Clark parts be up to those standards wouldn't mean anything. Therefore, it must be true that the government standards have some effect for the argument to stand.
Answer (E) is incorrect because it is slightly off topic. This argument isn't about parts made in this country, but rather is about Clark parts specifically. Even if the other parts made by Brands B and C are bad, it is still possible that Clark parts are good.
