The correct answer is: "Britain".
Britain is located in an archipelago in the Northern Atlantic Ocean and hence the sea consitituted a natural barrier. While the ground forces from nazi Germany were able to conquer many territories in continental Europe during the first two years of WWII (France, Belgium, the Netherlands, Poland) and they even managed to threaten the vast Soviet Union territories and to arrive 40 km away from Moscow, they were never able to exercise control over British soil during WWII.