As a result of losing the 100 Years' War, the English lost all of its territory in mainland Europe. This includes regions such as Gascony, Normandy, and Aquitaine that were previously under English control but were eventually ceded back to France. This loss significantly reduced English territorial holdings on the continent.
It is important to note that the English did not lose all overseas territories in China, as China was not directly involved in the 100 Years' War. Similarly, the English did not lose several ports in Africa as a direct consequence of losing the war.
Therefore, the correct answer is:
A. All of its territory in mainland Europe
