Final answer:
The annexation of Austria by Germany, or Anschluss, was greatly influenced by a sense of German nationalism felt by Austrian citizens, alongside Nazi propaganda pushing for unification.
Step-by-step explanation:
The factor that helped Germany annex Austria, known as the Anschluss, was D. Austrian citizens felt an increasing sense of German nationalism before World War II. The annexation in March 1938, facilitated by Adolf Hitler, capitalized on the widespread nationalism and a favorable view of unification with Germany amongst the Austrian people, which was influenced by Nazi propaganda. Despite the chaotic nature of the German pseudo-invasion, many Austrians welcomed the German troops, showing a form of consent to the annexation. The presence of German-speaking populations in Austria gave Hitler pretext to annexation, claiming the right to reunite German-speaking peoples under the Third Reich.