The fact that only one jumbo-jet manufacturer remains in the United St

The question asks for the option that best explains why the domestic jumbo-jet manufacturer raised the national defense issue in its campaign for import quotas.

The domestic manufacturer likely used this argument to gain public or governmental support for the quotas. We need to find the answer that shows why the national defense argument was particularly useful or necessary for the manufacturer.

Analyzing the Options:
(A) When airlines order new jumbo jets, they provide a large amount of work for aircraft manufacturers.

This is a general fact about the industry and does not explain why the manufacturer specifically used the national defense argument.
Eliminate.
(B) The President is not very concerned with the impact of international competition on the domestic jumbo-jet industry.

If the President is not concerned about international competition, the manufacturer might need a stronger argument (like national defense) to gain attention. However, this doesn't directly explain why the manufacturer raised the issue itself.
Eliminate.
(C) The manufacturer encountered resistance to public acceptance of import quotas on jumbo jets when it cited other rationale besides the national defense argument.

This explains that other arguments for import quotas were not effective, so the manufacturer switched to the national defense argument to gain support.
Keep.
(D) Most of the military's operations do not require the use of jumbo jets.

This weakens the relevance of the national defense argument rather than explaining why the manufacturer used it.
Eliminate.
(E) The domestic manufacturer recently received several orders from both airline and military sources, and these orders will allow that firm to compete successfully with its foreign competitors.

If the manufacturer is already receiving enough orders to compete, it would not need to argue for import quotas. This weakens the reasoning behind raising the national defense issue.
Eliminate.