The physical geography of the area significantly influenced the areas the Mongols were able to occupy. Among the options provided, d. Russia is particularly notable. The vast steppe regions of Russia facilitated the mobility of Mongol horsemen and their armies, allowing them to expand rapidly across the territory. The relatively open terrain contrasted with the mountainous and forested areas that might have posed logistical challenges.
While the Mongols did manage to invade and conquer parts of China and Persia, those regions had more varied geography, including mountains, rivers, and fortified cities, which presented more significant challenges to conquest compared to the expansive steppes of Russia. Japan, on the other hand, was never successfully invaded due in part to its geographical isolation as an island nation and the Mongols' difficulties with naval expeditions.
In summary, while the Mongols interacted with all four regions, the physical geography of d. Russia was particularly conducive to their methods of warfare and expansion.