The correct answer is "weapons."
The industry that was used as an example of protecting national security is weapons.
The United States government is so interested in strategic industries for all that they represent for the American people. These are strategical industries that are protected by the US government for considering them of national security interests. Weapons is one of them because it represents the industry that produces the weaponry, airplanes, tanks, and technology to defend the United States.
This also represents a good trade industry that exports weapons to other countries, generating an important income for the US and creates thousands of jobs.