Final answer:
The theory that all the continents were once a single landmass is supported by evidence such as the fit of the continents, matching fossils, and matching mountain ranges.
Step-by-step explanation:
The theory that all the continents were once a single landmass is supported by several lines of evidence. First, there is the fit of the continents, which can be observed by looking at a world map or globe. Specifically, the coastlines of North and South America can be aligned with the coastlines of Europe and Africa, suggesting that these landmasses were once connected. Second, there is the presence of matching fossils on separate continents. For example, similar fossils have been found in South America and Africa, indicating that these continents were once joined. Finally, there is the presence of matching mountain ranges on separate continents. This suggests that these landmasses were once connected and have since drifted apart.