Final answer:
The Kerch Strait links the Black Sea's northern extension to the Sea of Azov, positioned south of the Russian Plain near the strategically important Caucasus Mountains.
Step-by-step explanation:
The northern extension of the Black Sea is linked by the Kerch Strait to the Sea of Azov. The Kerch Strait serves as a critical connection between these two bodies of water and is of strategic importance to the region. It is situated to the south of the Russian Plain, where the Caucasus Mountains form a natural boundary between Europe and Southwest Asia. Moreover, the Black Sea is further connected through extensive network of canals and waterways, as the Volga River flows into the Caspian Sea and a canal links the Volga with the Black Sea through the Don River. This illustrates the interconnectedness of water bodies in Southern Russia and their importance for transportation and trade.