The correct answer is D) it was not right for the United States to conquer other people.
The best argument against American imperialism was that it was not right for the United States to conquer other people.
Another good argument could be that imperialistic ideas cannot be found in the US Constitution and is not part of the principles the founding fathers established for the United States.
It was the idea of the "Manifested Destiny" of 1845, who spread the belief that the United States deserved to expand its dominion and presence to other territories with the help of God.