The correct response is:
The mechanism (the proof) as to why continents were moving in the first place.
When Alfred Wegener proposed the Continental Drift Theory, he provided substantial evidence supporting the idea that continents were once connected and have since drifted apart. However, the scientific community was skeptical primarily because Wegener could not provide a convincing mechanism for how the continents moved. His ideas about the forces behind continental movement were not supported by the scientific understanding of geology and physics at the time. It wasn't until the development of plate tectonics in the mid-20th century, which provided a solid mechanism for continental movement, that his ideas gained widespread acceptance.