The first statement is the most accurate comparison:
"The Western Front had flat terrain, whereas the Eastern Front had mountains, valleys, rivers, and forests."
The Western Front, primarily located in France and Belgium, was characterized by trench warfare on relatively flat terrain. In contrast, the Eastern Front, which extended from the Baltic Sea in the north to the Black Sea in the south, featured a more varied landscape, including mountains, valleys, and forests.
The other statements contain inaccuracies regarding the geographic locations and characteristics of the fronts.