The correct answer is D) transportation networks.
What was targeted in the air campaign launched against North Vietnam was transportation networks.
We are referring to the "Operation Rolling Thunder" in North Vietnam. Having passed the "Gulf of Tonkin Resolution," the President had authorization of the US Congress to use the Army in Vietnam. So, United States President Lyndon B. Jhonson ordered "Operation Rolling Thunder" on February 13, 1965, to immediately start the air raid over Noth Vietnam strategic points such as transportation networks.