The correct answer is D. The route followed flat terrain useful to soldiers.
Explanation
According to the map, it can be seen that the armies of the crusades from the city of Regensburg (Germany) used a large part of the course of the Danube River located in central Europe to travel to Constantinople. This river was born in Germany and flows through countries such as Austria, Slovakia, Gungría, Croatia, Serbia, Romania, Bulgaria, Moldova, and Ukraine, between the mountain ranges of the Dinaric Alps and the Carpathian Mountains until it empties into the Black Sea. According to the map, you can see that the crusade from Germany, once it reached the mouth of the Black Sea, headed south to go to the city of Constantinople. According to the above, the correct answer is D. The route followed a flat terrain useful for soldiers.
