Question

according to the constitution, the united states can declare war ....?
when a majority of the states vote for war,
only by an act of congress.
when the president thinks it is necessary.
only after the u.s. is attacked.

Explanation:

The U.S. Constitution grants Congress the power to declare war (Article I, Section 8). State votes don't determine war declarations, the President can't unilaterally declare war (they are Commander - in - Chief for conducting war), and the U.S. can declare war in other scenarios too (not only after attack, e.g., WWII declaration before attack on Pearl Harbor was known). So the correct option is the one about an act of Congress.

Answer:

B. only by an act of Congress.