The best response that explains how Hitler was able to build up the German army despite the restrictions of the Treaty of Versailles is:
He took advantage of appeasement and a lack of force behind the League of Nations.
This response highlights how the international community, particularly Britain and France, were unwilling to confront Hitler's rearmament due to policies of appeasement and the ineffectiveness of the League of Nations, allowing him to rebuild the military with little opposition.