The correct answer is D) Britain and other Allied countries purchased and transported military weapons and equipment.
The United States supported the Allies during the Battle of Britain in that Britain and other Allied countries purchased and transported military weapons and equipment.
The Battle of Britain started on July 10, 1940, and ended on October 31, 1940, with a sounded victory of the Royal Air Force over the Germany Air Force. At that moment, the United States maintained the foreign policy of neutrality and did not directly participate on the allies' side, however, it sold weaponry and sent supplies to the war front.