Final answer:
The Yenisey River is the fifth-largest in the world, originates from Mongolia, and empties into the Yenisey Gulf in the Kara Sea.
Step-by-step explanation:
The river in question, which is the fifth-largest in the world and centrally located among the three great Russian rivers, is sourced from Mongolia and flows into the Yenisey Gulf in the Kara Sea, is the Yenisey River. The Yenisey is ranked as one of the largest rivers on earth by discharge and serves as a significant part of Russia's natural landscape. The closest contenders, but not matching the description, are the Irtysh, Ob, and Lena rivers, which are also major rivers flowing through Siberia to the Arctic Ocean. However, only the Yenisey fits the full description given in the question.