B. Tonkin Gulf Resolution
The Domino Theory is the idea that if one country falls to communism than a domino effect will occur and neighboring countries will become communist. This thought was a large player in causing the Vietnam War but it is not a resolution and it did not begin the bombing campaign. Operation Rolling Thunder is the name of the actual bombing campaign but it is also not a resolution. Lyndon B. Johnson was the President during that time and the one who carried out the bombing campaign but once again, Johnson was not a resolution. The Gulf of Tonkin Resolution was passed by Congress and authorized the use of the military in Vietnam, leading to the bombing campaign.
Hope I could help!
Also your answer choices are out of order (ACBD).