The right answer is united states. Given that the War was not specified let us take a look on both wars. In both wars USA was the last to join the allied fighting force, because of its neutrality policies and its geographic neutrality.
WWI: The USA came into war, on the side of the Triple Entente (or Allies), in April 1917, after Germany had resumed its unrestricted submarine warfare. By this time, France and Britain were already at war.
WWII: The USA declare war on Japan on December 8, 1941 after the attack of Pearl Harbor. Putting an end in the American neutrality again. And again, France and Britain were already at war.
Among the choices provided above the country that last to join the allied fighting force is the United States who joined the fighting force in December 1941 after<span> the </span>Japanese attack on Pearl Harbor<span>. Thank you for posting your question. I hope it helps. </span>
