When the german air force bombed great britain, the british were able to hold them off during the battle of britain because of their question 4 options: more powerful air force radar and message decoding technologies advanced satellite systems secure subway system that had been reinforced with steel

More powerful air force radar and message decoding technologies.

The Germans made efforts to bomb Royal Air Force airfields and take out Britain's radar defense system, but the outnumbered RAF fighters managed to outmaneuver their opponents, aided by radar technology.

As for message decoding technologies, the code-cracking machine developed by Alan Turing had an impact particularly on naval battles in the Atlantic between the Germans and the Allies.  Once they developed the ability to decode German messages, the British had to proceed carefully and not act on every military message or the Germans would have suspected their encoded messages were being hacked.

More powerful radar system contributed to the success of The royal air force to the german fleetline. it is important to note that despite heavy casualty on the German side after the france war, Hitler embarked on the futile campaign,one of the most fought air battle in history. the royal forces maintained air superiority in the battle and downed over 2000 German planes.

The answer is ethnic groups.