European leaders of England and France were slow to stop the advances of Hitler and Mussolini because

Athey believed Hitler was harmless.
Bthe League of Nations prevented them acting against Hitler and Mussolini.
Cwith the European economy booming, they failed to take the threat seriously.
Dthey were still reeling from World War I and hoped to avoid another large-scale conflict.

Verified Answer :

Neuron

European leaders were slow to stop the advances of Hitler and Mussolini because they were stil reeling from World War I and hoped to avoid another large-scale conflict - D. Despite saying this, the European nations had a sense of what was to come, only that they didn't really think it would come at such a vast price and body count tag.

THE ANSWER IS D.THEY WERE STILL REELING FROM WORLD WAR l AND HOPED TO AVOID ANOTHER LARGE- SCALE CONFLICT

  TRUST ME I TOKE THE TEST

                                                                                  HOPE I HELPED!