Correct answer: A. The US had suspicions that Iraq was hiding weapons of mass destruction.
Note: Answer B is not correct. The US knew that the 9/11 attacks had been carried out by terrorists that had been living in Afganistan, and the US began a war effort in Afghanistan to go after those terrorists.
Context:
In addition to the overtly stated reason about "weapons of mass destruction" as a cause for the war in Iraq, there was also the underlying "Bush Doctrine" of the value of a "preemptive strike" against a dangerous regime. The Bush Doctrine (after Pres. George W. Bush) proposed that the best defense against terrorism was to use American power to spread democratic values in countries that were potential breeding grounds for terrorist activity.
This sort of policy agenda was part of the "neoconservative" view of a number of President George W. Bush's advisers -- especially some who had also served in the administration of his father, President George H.W. Bush. In the wake of the 9/11 attacks, there was a desire to push American values and not be shy about doing so with the use of American military might. The core ideas of the "Bush Doctrine" were that the United States could pursue this goals on its own (without need for United Nations partnerships), that preemptive strikes were allowable against countries that harbored terrorists, and that regime change for the sake of promoting democracy was a good strategy.
